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Major Events in the History of Life


Abstract

This page provides a high level review of some of the main threads of the evolution of plants and animals, from the origin of life to the present day.

Keywords: evolution, metazoan radiation, Ediacaran, Cambrian, Embryophyta, tetrapods, Permian, extinction, dinosaurs, angiosperms, mammals, humans

Acknowledgement: This page owes both its inspiration and title to the excellent book edited by J.W. Schopf.

Introduction

Life on earth has a long and rich history – dating back almost to the time when the Earth’s crust solidified. This was a time when the planet was unrecognisable; when catastrophic bombardment by meteors doubtless wiped out the first microbes to evolve, again and again.

Much of this earliest life was microscopic and, microscopic or not, virtually unknown until the 1960s. Prior to then, as far as was widely known – even among the scientific community – the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and marvellous diversity: the so-called Cambrian Explosion. Whether the “explosion” was real or an artifact of preservation, by the end of the Cambrian all readily fossilisable modern animal phyla except the bryozoans (Entoprocta + Ectoprocta) were represented in the fossil record.

Plants had existed since the earliest fossils were formed, but did not achieve a high degree of organisation until vascular plants evolved on land, perhaps around the Silurian.

The chordates were established and diversified throughout the Early Paleozoic, giving rise to fishes and, later, amphibians, reptiles, dinosaurs, birds and mammals – eventually man.

 
 

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Further Reading

Related Pages

Coming later this year

  • Highlights in the Evolution of Plants
  • Highlights in the Evolution of Vertebrates

Other Web Sites

 
 

Origins and Early Development

As recently as the 1960s, as far as was widely known, the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and marvellous diversity: the so-called “Cambrian Explosion” (see below). Indeed, Charles Darwin (Origin of Species – 2nd ed. Chapter IX) recognised that the sudden appearance of animal fossils in the Cambrian posed a problem for his theory of natural selection. He suggested that “before the lowest [Cambrian] stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the [Cambrian] age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures.”

He was quite correct, on both counts.

Oldest Fossils

The oldest rocks known are those of the Isua Supracrustal Group of southwestern Greenland, surviving from around 3700 Ma but, unfortunately, preserving no fossils. The Isua rocks are strongly metamorphosed; although some sequences have been demonstrated to be of sedimentary origin, and may have once contained fossils, the heat and pressure to which they have been subjected will have destroyed them. Isotopic signatures of carbon recovered from these rocks provides indirect evidence that life may have existed in these remote times (Rosing 1999, Brack 2002). Van Zuilen et al. 2002, however, advises approaching this interpretation with extreme care, noting that graphite occurs abundantly in secondary carbonate veins formed at depth by injection of hot fluids reacting with older crustal rocks.

The oldest fossils known to date, derive from the Apex Chert, a formation of the Pilbara Supergroup occuring in northwestern Western Australia, and dated at 3,465 Ma (± 5 Ma, see Schopf 1999, pp. 88-89). However, the fossils occur in fragments of rock within the chert; thus they are even older, though by how much, is unknown. The organisms themselves are filamentous, composed of distinct, organic walled cells occurring as a uniserial string, and are interpreted as cyanobacteria.

Also occurring in the Pilbara Craton of western Australia, the Strelley Pool Chert (SPC) includes structures interpreted as stromatolites. A 2006 study of these concludes that the evidence “strongly indicates that organisms flourished on a broad peritidal platform 3.43 Gyr ago in the Pilbara, rapidly taking hold and creating a reef-like build-up in shallow waters as surfaces became submerged. The variety of stromatolites present indicates that the SPC may contain not only some of Earth's earliest fossils but also a diverse fossil 'ecosystem', sustained by shallow seawater free of terrigenous influx - ideal conditions for phototrophism” (Allwood et al. 2006, pp. 717-8).

Another ancient cyanobacteria collection is reported from the Nauga Formation, Prieska, South Africa, between 2,588 ± 6 and 2,549 ± 7 Ma (Kazmierczak & Altermann 2002).

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Fig 1. A rRNA (16S) cladogram showing presumed phylogenetic realtionships between the three superkingdoms: bacteria (yellow), Archaea (archaeans, green) and Eucarya (eukaryotes, blue). (After Schopf 1999, fig. 4.2.) However, eukaryote genome analysis is not necessarily straight forward; “it has been established that the eukaryotic genome is a chimaera where genes of ancient eukaryotic ancestry coexist with genes more recently acquired from bacterial endosymbionts” (Brown 2002).

The occurrence of cyanobacteria of this age is consistent with the belief that aerobic metabolism evolved independently in all three lineages, after the diversification of the superkindoms, yet prior to the widespread introduction of molecular oxygen into the anaerobic biosphere at approximately 2.2 Ga. “The fact that enzyme distribution in aerobic pathways was largely incongruent with organismal speciation suggests that adaptation to molecular oxygen occurred after the major prokaryotic divergences on the tree of life. This is supported by data from geological and molecular evolutionary analyses, showing that all three domains of life, and many phyla within these domains, had appeared by the time that oxygen became widely available.... The relatively late onset of atmospheric oxidation argues against the invention of O2-dependent enzymes or pathways in the last common ancestor of modern organisms, suggesting that adaptation to molecular oxygen took place independently in organisms from diverse lineages exposed to O2” (Raymond & Segrè 2006).

At 2,000 Ma we find possible eukaryotes, and by 1,750 Ma we are confident of this identification. The current “standard” view of the origin of eukaryotes, revolutionary in its time, is Margulis’ endosymbiont hypothesis, which describes how eukaryotes improved their metabolic capacity by engulfing certain prokaryotes and converting them into intracellular organelles, principally mitochondria and chloroplasts.

“The bacterial endosymbiosis which led to the development of the mitochondria probably occurred very early in eukaryotic evolution. Phylogenetic analyses using different molecular markers place at the base of eukaryotes various protist lineages (the archamoeboe, metamonads, microsporidia and parabasalia) known as the Archezoa. The Archezoa are not a true clade of organisms but rather a collection of species united by their lack of mitochondria, anaerobic metabolism, simple cell morphology and bacteria-like ribosomes” (Brown 2002).
“However, recent molecular phylogenetic studies are now showing that all major groups of the Archezoa either secondarily lost their organelles or underwent some kind of endosymbiosis which resulted in the successful fixation of several bacterial genes in the Archezoan nuclear genome but not the retention of an intracellular organelle. ... This would suggest that endosymbiosis occurred very early in eukaryotic evolution, prior to the emergence of the Archezoa or any known eukaryotic species. Indeed, the harnessing of a bacterial endosymbiont for energy production, and possibly other uses, might have been the defining event in the formation of the eukaryotic cell” (Brown 2002).

(Read more about Archezoa.)

Emergence of the Eucaryote Kingdoms

“An increasingly well resolved Proterozoic fossil record documents the late Mesoproterozoic to early Neoproterozoic presence of most of the major clades (kingdoms) of eukaryotes, including the rhodophytes, stramenopiles, alveolates and green plants. A coincident rise in acritarch diversity, combined with molecular phylogenetic evidence for rapid cladogenesis, points to a major radiation of eukaryote groups at this time, sometimes referred to as the ‘big bang’ of eukaryotic evolution. Bangiomorpha pubescens, from the 1200 Ma Hunting Formation, arctic Canada, is the earliest taxonomically resolved eukaryote on record. More importantly, it is the earliest known example of both sexual reproduction and complex multicellularity. The introduction of differentiated multicellular organisms would have had profound implications for contemporaneous ecology (e.g. with its differentiated basal holdfast Bangiomorpha was able to anchor itself in the substrate and orient itself vertically – the first instance of eukaryotic tiering – which would in turn have induced new environments and evolutionary opportunities). Buss (1987) has presented compelling arguments for why sex is a necessary prerequisite to complex multicellularity (by enabling the expulsion of somatic cell parasites), and it is clear that complex multicellularity is the source of almost all organismal morphology. I suggest that the principal significance of the evolution of sex was the ‘invention’ of organismal morphology, and thereby the directional, escalatory and ‘progressive’ evolution of a biological environment. The modern eukaryotic kingdoms would appear to be the consequence of that first indulgence” (Butterfield 1999).

By 1,100 Ma we find a rapid diversification of planktonic eukaryote assemblages, followed by an inexplicable decline in both abundance and diversity, between 900 and 675 Ma.

Around 605 to 590 Ma, the Varanger glaciation occurred; possibly the most profound planet-wide glaciation in our history. During the event, the earth biota suffered its first “mass extinction” - primarily of acritarchs.

Grypania spiralis

Grypania spiralis was a coiled, spaghetti-like organism up to half a metre in length found in ~1300 Ma shales and slates from Montana, China and India.

It is not known for certain what kind of organism this fossil represents, to which (if any) of the moden kingdoms it is most closely related, nor even if it was a large unicell – perhaps with many nuclei – or multicellular.

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Fig. 2: Grypania spiralis (Walcott) Walther et al. 1976 emend. Walther et al. 1990 [Image coutesy of Dr. Bruce Runnegar, Center for Astrobiology, Institute of Geophysics and Planetary Physics, UCLA, Los Angeles.]

First Metazoans

“Metazoans (multicelled animals) appear abruptly in the fossil record at the close of the Precambrian. Although this striking event has been obvious since early in the study of historical geology and has attracted a good deal of scientific attention during the last decade, several fundamental aspects of the early history of the Metazoa remain unclear. For example, are metazoans a monophyletic group derived from a single or unicelled organism? If so, are metazoans descended from cilliated unicells, from other kinds of unicells, or perhaps from multicellular eukaryotes such as fungi or plants? Was the Precambrian history of the Metazoa relatively long (for example, 500 million years) or relatively short (less than 100 million years)? Were late Precambrian soft-bodied organisms, collectively known as the Ediacara fauna, fundamentally different from Paleozoic and younger metazoans as Seilacher (1989) has suggested; and, if so, was there a mass extinction at the end of the Precambrian? And finally, how and when did the major higher taxa (phyla) of living animals evolve?” (Runnegar 1992, p. 65).

Earliest Trace Fossils

Seilacher et al. (1998) reports trace fossils from the one billion year old Chorhat Sandstone formation in the Son Valley, central India, which the authors interpret as the burrows of triploblastic undermat miners - infaunal animals that excavated tunnels underneath microbial mats “which served as a food source as well as an oxygen mask for the wormlike animals that were exploiting its decaying base” (Seilacher et al. 1998, p. 81).

If these traces are true fossils, they provide evidential support for the claim of molecular biologists that triploblastic metazoans existed twice as long ago as the assumed Cambrian evolutionary explosion of modern animal phyla. However, the trace fossil interpretation of this find is by no means unquestioned.

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Fig. 3: Reproduction of fig. 2 from Seilacher et al. 1998, showing  probable worm burrows from the Mesoproterozoic Chorhat Sandstone of central India. A remnant of the original sand veneer cover (arrow) shows that the burrows are not modern artifacts. (Yale specimen YPM 37665; photograph by W. Sacco.)

Earliest Body Fossils

At approximately 610 to 600 Ma, circular impressions from the Twitya Formation of the Mackenzie Mountains provide evidence for the earliest metazoans, simple cup-shaped organisms, possibly cnidarians.

Somewhat later, perhaps 590 to 565 Ma, but still predating the classic Ediacaran assemblages, the Doushantuo phosphate deposit in China is slowly yielding a surprisingly diverse biota, including probable algae, sponges, cnidarians and bilaterians (read more).

Both morphological and molecular analyses indicate that the bilaterians are monophyletic, though it is unknown how deep in time lived the latest common ancestor from which arose the two great bilaterian clades, the protostomes and deuterostomes.

Subsequently, “[t]wo distinct evolutionary pulses, represented by the Vendian Ediacaran fauna and Cambrian small shelly faunas, are generally thought to characterize the emergence of macroscopic animals at the end of Precambrian time. Biostratigraphic and uranium-lead zircon age data from Namibia indicate that most globally distributed Ediacaran fossils are no older than 549 million years old and some are as young as 543 million years old, essentially coincident with the Precambrian-Cambrian boundary” (Grotzinger et al. 1995, Abstract).

The Ediacaran Assemblage

The Ediacaran assemblage is an assemblage of distinctive but poorly understood marine fossils occurring, typically, in late Vendian times, but persisting into the basal Cambrian. The assemblage predates by a distinct interval of perhaps 20 Ma or more, the so-called “Cambrian Explosion” when ‘modern’ multicellular life began to diversify rapidly. Occurences have been located on every continent except Antarctica.

For some years a number of authors (e.g. Seilacher 1984, 1989; McMenamin 1986) were able to argue, convincingly, that the Ediacarans were unrelated to any living group of organisms; that they represented a new kingdom (Vendobionta Seilacher 1992) which was wiped out by a mass extinction event at the Vendian-Cambrian boundary. However, much of the ‘evidence’ supporting this view has been over-turned by the discovery of Edicaran assemblages persisting into the Cambrian and of morphologically similar forms which clearly are animals. This view has now lost much of its cogency.

(Read more about the Ediacaran Assemblage.)

Small Shelly Faunas

“Small shelly fossils near the base of the Cambrian mark the transition to a skeletonized fauna and the metazoan-dominated Phanerozoic fossil record. The recovery of articulated specimens composed of multiple sclerites discussed above, such as Wiwaxia, Halkieria and Microdictyon, suggests that much of the remaining ‘small shelly fauna’ represent elements similarly employed as ectodermal armor in bilaterian Metazoa that have yet to be recovered in an articulated form. In addition, recent finds of cap-shaped shells composed of fused spicules of the early Cambrian age (Bengtson 1992) support an argument of fusion of individual skeletal elements to form sclerites, plates, or shells (Haas 1981)” (Jacobs et al. 2000, p. 345).

(Read more about the small shelly faunas.)

(A) Spriggina


















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Fig. 4: (A) Spriggina floundersi Glaessner – From the Vendian Pound Quartzite of the type locality, Ediacara, South Australia. Overall length about 10 cm. Specimen from the Yale collection (YPM 63257).

(B) Dickinsonia costata Sprigg – Vendian, from the Brachina Gorge, Flinders Ranges, South Australia. Specimen from the Yale collection (YPM 35467). Dickinsonia has been known to reach dimensions of up to a metre.
[Images courtesy of the Peabody Museum of Natural History, Yale University.]

Eucaryote Diversification – The Phanerozoic

The Cambrian Metazoan Radiation

In the sediments of Early Cambrian age, fossils ‘suddenly’ become common for the first time. “Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla” (Wray et al. 1996). This effect, which has come to be known as the “Cambrian Explosion,” seemed even more pronounced prior to the 1950s when older fossils were all but unknown. Since then, the discovery of the Vendian age Ediacaran assemblages, probable metazoans perhaps 600 Ma (e.g. those of the Doushantuo phosphates), and the far more ancient microfloras from the Gunflint and Apex Chert Formations, have somewhat diminished the effect. Nevertheless, it appears that a genuine phenomenon remains.

Genetic evidence suggests that many if not most of the taxa first appearing as fossils in Cambrian sediments had actually evolved earlier (though just how much earlier is still very much a subject for debate – read more) and that the Cambrian ‘explosion’ may be a taphonomic artifact, possibly, due to many diverse groups acquiring more readily fossilised calcareous skeletons at more or less the same time. This last idea is not as improbable as it first sounds: for example, steadily rising levels of atmospheric oxygen may at that time have passed some important threshold, thereby enabling a new biochemical pathway for all organisms, simultaneously.

However, other genetic research leads to different conclusions. For example, “... engrailed data reported here, in combination with previous scenarios for the formation of ectodermal armor and recent fossil discoveries, suggest a singular evolution of invertebrate skeletons near the base of the Cambrian, followed by subsequent loss in several lophotrochozoan and ecdysozoan lineages. Such an interpretation, if substantiated, would have important implications for the Cambrian radiation, as it would constrain readily fossilizable exoskeletons to a single lineage, leading to a monophyletic clade of bilaterians. This would lead to a closer association of the bilaterian diversification with the base of the Cambrian” (Jacobs et al. 2000, p. 345).

Discussion of early metazoan evolution has for many years been dominated by fossil evidence from the Middle Cambrian Burgess Shale and, in particular, by its famous problematic arthropods – Anomalocaris, Leancoila, Opabinia, and so on. “No one would dispute that these fossils are problematic, in the sense that they are difficult to understand. However, that methodological difficulty should not be confused with the possibility that these fossils have only remote affinities with all living groups” (Budd 1997, p.125).

(Read more about the Cambrian and the Cambrian Explosion.)

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Fig. 5: Reproduction of fig. 2 from Wray et al. 1996 showing their estimated divergence times for selected metazoan phyla, based on seven genes, with standard errors indicated by shaded bars. The three estimated divergence times nest in agreement with well-corroborated phylogenetic relationships, but their antiquity is by no means accepted by all researchers. Note that the chordate-echinoderm and chordate-protostome divergence times are significantly different from each other. Divergence times among the three protostome phyla were not estimated in their analysis.

Trilobites

For many, the real ‘stars’ of the Early Paleozoic are the trilobites, extinct, exclusively marine arthropods, known fondly to everybody who has read much about fossils. They appear suddenly in great abundance in the Cambrian, and evolved into a wonderful diversity during the early Paleozoic, before gradually dwindling away towards extinction in the Permian. They are among the most distinctive index fossils of their time, ranging in size from a few millimetres (Agnostus) to more than 60 cm (e.g. the Devonian form, Terataspis).

Trilobites are clearly arthropods. They exhibit a number of primitive arthropod features which, together with their very early appearance in the fossil record, have sometimes led to trilobites being considered to lie close to the main arthropod ancestral line. On the other hand, the trilobite eye is highly derived – there is nothing else like it in the arthropod (or any other) lineage – which belies this placement. Cladistic analyses based on morphological features (e.g. Wills et al. 1998) place trilobites, together with Naraoia and Molaria, and adjacent to several other exotic Burgess Shale forms, within the Arachnomorpha.

The earliest trilobite representatives – from the order Redlichiida and in particular the Fallotaspididae – are distinctly and emphatically trilobites, and they do not look like anything else. They provide few clues to which other arthropod groups may be their close relatives, or to their origins. Although it is true that one or two of the Ediacaran forms such as Spriggina (fig. 4A) and “Archaeaspis” fedonkini superficially resemble early trilobites, to date the detailed case for such an ancestry is far from compelling.

(A) Redlichia chinensis (21254


















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Fig. 6: (A) Redlichia (Hunanolenus) chinensis – Cambrian. Specimen from Henan Province, China.  Length approximately 6 cm. [Original image.]

(B) Paciphacops raymondi – Phacopina. A common fossil from the famous Lower Devonian Haragan Formation, Coal Co., Oklahoma. Typified by the extremely inflated glabella which protrudes forward over the anterior edge of the cephalon. Specimen approximately 3 cm. in length. [Original image.]

The Chordates

The origins of chordates are still relatively obscure: “Tunicates, or urochordates, comprise the most basal chordate clade, and details of their evolution could be important in understanding the sequence of character acquisition that led to the emergence of chordates and vertebrates. However, definitive fossils of tunicates from the Cambrian are scarce or debatable. Here we report a probable tunicate Cheungkongella ancestralis from the Chengjiang fauna. It resembles the extant ascidian tunicate genus Styela whose morphology could be useful in understanding the origin of the vertebrates” (Shu et al. 2001, p. 472).

Cephalochordate-like animals (most famously Pikaia from the Middle Cambrian Burgess Shale but perhaps more significant is Cathaymyrus from the Lower Cambrian Chengjiang lagerstätte) have been known for some time. The discovery of two distinct types of agnathan (Subphylum Vertebrata, Class Agnatha) from the Chengjiang at Haikou (Shu et al. 1999) indicates an unexpected vertebrate diversification early in the Cambrian. Rather surprisingly, Shu et al. conclude there is “no reason to suppose that the origin of vertebrates was hundreds of millions of years earlier” (somewhat mischeviously misrepresenting Bromham et al. 1998) with the clear implication that the vertebrates, at least, “exploded” within sight of the Cambrian. However, although nothing resembling a chordate is known from Ediacaran assemblages, evidence of a long Precambrian history seems to be accumulating for most Cambrian forms.

“The very earliest conodonts are known from rocks of probable Precambrian age in Siberia, they are found more commonly in Cambrian deposits, diversity increased in the Ordovician and again during the Devonian. The conodont-bearing organism clearly survived the Permo-Triassic boundary extinctions but became extinct during the late Triassic. It has been noted that the extinction of the conodonts coincides with the diversification of dinoflagellates and first appearance of calcareous nannofosils. The most primitive conodonts are single cones, which dominate early Ordovician assemblages and reach a peak in the Arenigian (late Early Ordovician). The first platform type conodonts occur around this time as well. Conodont diversity and abundance declined in the Silurian. During the early and mid Devonian diversity gradually increased, reaching an acme in the late Devonian. In the early Carboniferous conodonts remained abundant and widespread but diversity decreased during the late Carboniferous. In the Permian the conodonts almost became extinct, however, they made a recovery in the early to middle Triassic only to disappear in the late Triassic” (University College London web site).

Jaws

“The acquisition of jaws is perhaps the most profound and radical evolutionary step in craniate history, after the development of the head itself. Jaws are not found in modern or fossil lampreys and hagfishes. Despite the fantastic variety of those armoured fishes, the ostracoderms, none of them seems to have had moveable internal jaw bones” (Maisey 1996, p. 59).
“The oldest identifiable gnathostome [jawed fishes; ® sidebar] fossils are extremely scrappy, consisting of isolated scales and teeth. Because paleontologists find similar scales and teeth in more recent fossils that are complete enough to reveal the presence of jaws, it is inferred that the early fishes from which these bony fragments came also had jaws. Among the most ancient scraps are supposed shark scales from the Silurian of Mongolia and the Ordovician of the USA, thought to be about 420 and 450 million years old respectively, resembling the simple skin denticles (small toothlike scales) of modern sharks” (Maisey 1996, p. 61).
The term gnathostome is generally reserved to mean the jawed fishes, though we should not lose sight of the fact that, from a phylogenetic perspective, the gnathostomes include all the jawed craniates, specifically the tetrapods. From a cladistic point of view, the jawed fishes are a paraphyletic group as opposed to a true clade; a pure cladist would not recognised the jawed fishes as a ‘natural’ group at all.
First to differentiate from the main gnathostome lineage were probably the armoured placoderms in the Ordovician, followed by the chondrichthyans (sharks and rays), also in the Ordovician. The remaining gnathostomes are known as Osteichthyans (bony fishes), and are characteristed by a swim bladder, which may be modified into a lung or lungs in more derived forms, a pericardial cavity for the heart, a brain that does not extend forward between the eyes, and several other apomorphies (Maisey 1996, pp. 120-121). The next group to become differentiated were the sarcopterygians (lobe-finned fishes and, later, the tetrapods). The oldest lobe-fin fossils are Early Devonian, though the group likely appeared earlier. Possibly last to differentiate from the main lineage leading to the ray-finned actinopterygian fishes, were the spiney-finned acanthodians, first appearing in the Silurian but becoming extinct in the Early Permian.
Elaboration of functional jaw structures in vertebrates not only allowed the transition from the simple body plan of the jawless agnathan fishes but also opened the door to evolution of complex skull and ear structures, as well as numerous specialized jaw and dentition structures. Jaw structures develop from the branchial arches, which are simple in agnatha and more complex in jawed gnathostomes.

Colonisation of the Land

Succession of Land Occupation by Plants

Although the invasion of the land by plants was undoubtedly a microcosm of evolution itself, accomplished in uncounted millions of incremental steps, it is usual to recognise four or five major overlapping phases (Gray 1985, Gray 1993, Edwards & Selden 1993, Kenrick & Crane 1997):
  • Microbial mats comprising prokaryotes and, later, photosynthesising protists on bare mineral surfaces, beginning in the Precambrian and still occurring today.
  • A bryophytic phase, from the Ordovician to the Early Devonian, evidenced by fossil spores and cuticles.
  • A “rhyniophytoid phase” (small vascular plants with an axial organisation and terminal sporangia such as Cooksonia) beginning in the Late Silurian.
  • Next was the first major diversification of land plant life, such as rhyniophytes, zosterophylls, drepanophycaleans, lycophytes, trimerophytes, and (in the southern hemisphere) Baragwanathia, beginning as early as Late Ludlow (Late Silurian) in Australia through Gedinnian (Early Devonian) on Laurentia.

The earliest record of a possible land plant is the Cambrian age Aldanophyton, unfortunately known from only a single occurrence in Russia. The original, and perhaps only detailed description is in Russian; information in English about this enigmatic fossil is difficult to come by. On the whole, this record must be regarded with some doubt.

"When [undoubted] land plants first appear in the fossil record, they are a mixture of simple and complex forms, hinting at a previous history we have not yet uncovered. though there are nonvascular plants (in other words, plants lacking tubes that carry nutrients and water) in Early Silurian deposits, no true vascular plants are known until the later part of this period" (Rich et al. 1996, p. 374).

=== Rhynie Chert ===

(Read more.)

Origin of True Land Plants

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Fig 7: Whereas most people, at least in the west, seem to have a broad appreciation of animal relationships, the same is not true of plants. Taking a phylogenetic (rather than a taxonomic) view of the kingdom Plantae, in broad terms it looks something like this. Gymn = gymnosperms, here shown as a paraphyletic group; Ang = angiosperms.

The embryophytes (land plants) have long been thought to be related to the green algal group Charophyta, though the nature of this relationship and the origin of the land plants have remained unresolved. Charophytes are generally rather unfamiliar although one genus, Nitella, is occasionally employed as a freshwater aquarium plant. Fossil charophytes are known from the Late Silurian, based almost exclusively on female fructifications, and reproductive structures of a bisexual charophyte have been reported from the Devonian of the French Massif Central (Feist & Feist 1997).

“A four-gene phylogenetic analysis was conducted to investigate these relationships. This analysis supports the hypothesis that the land plants are placed phylogenetically within the Charophyta, identifies the Charales (stoneworts) as the closest living relatives of land plants, and shows the Coleochaetales as sister to this Charales/land plant assemblage. The results also support the unicellular flagellate Mesostigma as the earliest branch of the charophyte lineage. These findings provide insight into the nature of the ancestor of plants, and have broad implications for understanding the transition from aquatic green algae to terrestrial plants” (Karol et al. 2001, p. 2351).

The roots of most land plants are surrounded by an underground cloud of fungal threads, called mycorrhizae. This is a mutualistic relationship in which the fungi provide minerals, which they acquire from the soil, in exhange for carbohydrates made by the plant. Colonisation of the land by plants might have been impossible without this association with mycorrhizae.

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Fig 8: Chara corallina, a stonewort and possibly one of the closest living relatives of land plants. The illustrated plant exhibits orange reproductive structures. [Image courtesy of Dr John Clayton, NIWA, Hamilton, New Zealand.]

Rustling in the Undergrowth*

The earliest evidence for terrestrial activity by animals is provided by trace fossils. Among the oldest are Late Cambrian to no younger than Arenig (Early Ordovician) tracks made by multiple ~50 cm-sized, many legged animals preserved in an eolian sandstone in the Nepean Formation (Potsdam Group) near Kingston, Ontario. However, these track-makers were probably amphibious arthropods - possibly euthycarcinoids - which only left the sea for a limited time, rather than fully terrestrial animals (MacNaughton et al. 2002).

Caradocian (Ordovician) trackways attributed to the ichnogenera Diplichnites and Diplopodichnus occur in the Borrowdale Volcanic Group of northwestern England. Such traces are consistent with tracks made by modern millipedes, although it is not possible to be certain that the trace fossils were left by fully terrestrial animals (Johnson et al. 1994).

Meandering, sub-vertical burrows in an Upper Ordovician paleosol from Pennslyvania show bilaterally symmetrical backfilling resembling that found today in burrows made in decaying wood by some millipedes.  Again it is impossible to confidently assign these burrows to millipedes or even to arthropods, but subaerial burrowing is strong evidence for a fully terrestrial habit by some animal.

The earliest terrestrial body fossils are unassignable fragments from the Llandovery (Lower Silurian) Tuscarora Formation near Millerstown, Pennslyvania. Acid digestion of rock samples has recovered pieces of bristles or setae which may be arthropodan although they do not appear to be hollow, as are nearly all arthropod setae. Annelid setae are not hollow (Gray & Boucot 1994).

The earliest identifiable terrestrial body fossils are arthropod fragments from the Late Silurian. However, these organisms possessed sensory and respiratory structures fully adapted for life on land, indicating an earlier history of terrestrial habituation that has not yet been found.

In contrast to megascopic plants, which appear to have colonized the land only once, many animal groups made the transition to terrestrial existence independently and overcame the problems of water relations in different ways. Early evidence for terrestrial animals is sparse, but by the Early Devonian exquisitely preserved arthropod faunas are known from several localities in North America, Germany and the United kingdom. These faunas document the appearance of diverse arthropod communities including centipedes, millipedes, trigonotarbids and their living relatives spiders, pseudoscorpians, mites (orbatids and endeostigmatids), arthropleurids (extinct arthropods), archaeognathans (primitive wingless insects), collembolans and possibly bristletails. Available evidence indicates that these animals were mainly predators and detritivores and, until the appearance of vertebrate herbivores in the latest Palaeozoic, most energy flow into animal components of early terrestrial ecosystems was probably through the decomposer pathway rather than direct herbivory. Indirect evidence for herbivory comes from wound responses in the tissues of some fossil plants, and perhaps also from fossil faecal pellets containing abundant spores (abridged from Kenrick & Crane 1997, p. 38).

* Some of the material in this section, including the nifty title, after Shear & Selden 2001.

Early Tetrapods

“The relationship of the three living groups of sarcopterygians or lobe-finned fish (tetrapods, lungfish and coelacanths) has been a matter of debate. Although opinions still differ, most recent phylogenies suggest that tetrapods are more closely related to lungfish than to coelacanths. However, no previously known fossil taxon exhibits a concrete character combination approximating the condition expected in the last common ancestor of tetrapods and lungfish—and it is still poorly understood how early sarcopterygians diverged into the tetrapod lineage (Tetrapodomorpha) and the lungfish lineage (Dipnomorpha). Here we describe a fossil sarcopterygian fish, Styloichthys changae gen. et sp. nov., that possesses an eyestalk and which exhibits the character combination expected in a stem group close to the last common ancestor of tetrapods and lungfish. Styloichthys from the Lower Devonian of China bridges the morphological gap between stem-group sarcopterygians (Psarolepis and Achoania) and basal tetrapodomorphs/basal dipnomorphs. It provides information that will help in the study of the relationship of early sarcopterygians, and which will also help to resolve the tetrapod–lungfish divergence into a documented sequence of character acquisition” (Zhu and Yu 2002, p. 767, Abstract).

“The fossil record of early tetrapods has been increased recently by new finds from the Devonian period and mid–late Early Carboniferous period. Despite this, understanding of tetrapod evolution has been hampered by a 20-million-year gap (‘Romer’s Gap’) that covers the crucial, early period when many key features of terrestrial tetrapods were acquired” (Clack 2002, p. 72). “This period between the end Devonian and the mid-Viséan represents the time when tetrapods underwent a major diversification and acquired true terrestriality” (Clack & Finney 1999).

“A nodule from the the Scottish Calciferous Sandstones, found near Dumbarton, western Scotland, preserves the only known articulated tetrapod material from ‘Romer’s Gap’” (Clack & Finney 1999).

The only articulated skeleton of a tetrapod yet found from the Tournaisian epoch (354–344 Ma) is Pederpes [® sidebar], “the earliest-known tetrapod to show the beginnings of terrestrial locomotion and was at least functionally pentadactyl” (??? ref).

With its later American sister-genus, Whatcheeria, it represents the next most primitive tetrapod clade after those of the Late Devonian, bridging the temporal, morphological and phylogenetic gaps that have hitherto separated Late Devonian and mid-Carboniferous tetrapod faunas” (Clack 2002, p. 72).

 
“This specimen is a nearly complete skeleton, lacking only the tail, some digits and the right side of the skull. It derives from the Ballagan Formation, Inverclyde Group, dated as the claviger-macra palynozone, Dinantian, Upper Tournaisian. The animal resembles the Viséan genus Whatcheeria in skull morphology, but differs in many postcranial features. Recently uncovered regions include the left stapes, a stout, stubby bone with a large stapedial foramen, resembling that of the Devonian Acanthostega. The ribs bear large triangular flanges, the vertebrae are rhachitomous, and the presacral vertebral count is about 27. There is a five-digit pes, but only two maneal digits are preserved, one short and stubby and the other slender. The humerus bears a spike-like latissiums dorsi process, resembling that of the baphetid genus Baphetes. This specimen should help resolve character polarities, phylogenetic relationships and the acquisition of terrestrial morphologies in early tetrapods” (Clack & Finney 1999).

The Permian Mass Extinction

There are a number of well-documented mass extinction events, notably occurring at the close of the Cambrian and Ordovician, within the Late Devonian, and at the close of the Permian, Triassic and Cretaceous Periods. Although the Cretaceous-Tertiary event is by far the most widely familiar – on account of its association with the ever-popular dinosaurs and Alvarez’ exciting platinum/iridium evidence for the role of a major meteor impact – the event at the end of the Permian was by far the most profound. At this time something like 60% of all species (and perhaps 90% of all marine forms) went extinct. The last few trilobites disappeared during this event although, as a group, they were already a very impoverished class by then.

=== something about the Siberian Trapps ===

No clear explanation is available and it is possible that the extinction was an emergent consequence of many factors which, individually, may have been too subtle to leave much evidence.

(Read more.)

Mesozoic Developments

Dinosaurs

Popular interest in the Mesozoic is largely focused upon the Jurassic and Cretaceous and essentially preoccupied with crown group dinosaurs. The interesting evolution, however, mostly occurred deep in the Triassic. By the end of this period, dinosaurs, pterosaurs, lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock.

If one can ignore the sheer spectacle of their size, the principal areas of interest in dinosaurs lie in their phylogenetic relationships with birds, the curiously lively and almost propagandist debate as to whether they were ‘warm blooded’ [® sidebar], and whether their final extinction was coincident with the platinum/iridium event at the Cretaceous-Tertiary boundary [® sidebar].

(Read more about dinosaurs, dinosaur myths and misinformation.)

Birds

Three possibilities for the origins of birds are still being credibly debated: The first is that they evolved from some unknown group of basal archosaurs, probably in the Triassic Period. Second, that they are a sister group to the Crocodylians, perhaps arising from within the sphenosuchian crocodylomorphs in the Early Jurassic. Certainly the most widely held view – though unfortunately misrepresented as the only credible modern view by the popular media – is for an ancestry among the theropod dinosaurs, specifically the Maniraptora, in the Middle to early Late Jurassic.

Despite intensive searching, the earliest known bird is still the famous Archaeopteryx, known from only seven skeletons and an isolated feather, all recovered from the Late Jurassic (Tithonian, perhaps 150 Ma) Solnhofen Limestone of Germany. The small theropod dinosaur, Compsognathus, has also been recovered from the Solnhofen. This fossil record represents a difficult problem for advocates of the theropod hypothesis: birds (specifically Archaeopteryx) are supposed to be most closely related to the dromaeosaurids, which do not appear in the fossil record until Albian times (mid Cretaceous, about 110 Ma) yet Compsognathus, which is believed to have diverged from the theropod lineage long before the evolution of the dromaeosaurids, occurs alongside Archaeopteryx 40 million years earlier. At present, only the vagaries of the fossil reord can be invoked to ‘explain’ the stratigraphic disjunction; our present understanding is unsatisfactory.

The first known beak and pygostyle (the “parsons-nose” which is all that remains in birds of the reptilian tail) occur in a Chinese fossil dated at 130 Ma. Feathers and bird bones have also been recovered from 110 Ma sediments in Victoria and Queensland.

Rahonavis is a primitive bird from 80 million-year-old rocks of Madagascar. Despite being more bird-like than Archaeopteryx, raven-sized Rahonavis retains some very distinctive theropod-like features. Other small primitive birds have been found elsewhere around the world. From Mongolia comes a large flightless bird, Mononykus, with wings replaced by a pair of single-digit hands that projected forwards. Another flightless bird is known from Patagonia. A sparrow-sized bird from Spain had a more modern shoulder joint than Archaeopteryx and a perching foot but it still had teeth.

Were the dinosaurs ‘warm-blooded’ (endotherms)?

Various sources would have one believe this question is effectively decided one way or the other whereas, in fact, the evidence either way is far from compelling. The co-existence of equally large marine reptiles* –  that were certainly cold-blooded – makes it quite clear that it was not necessary for large, Late Mesozoic animals to be warm-blooded, but it doesn’t necessarily mean they weren’t either.

* For example, Liopleurodon was a euryapsid reptile (Class Reptilia, Order Plesiosauria) about 25 metres long; the size of a large dinosaur such as Apatosaurus.


When did the dinosaurs become extinct?

Dinosaurs were certainly very much on the decline towards the end of the Cretaceous; they may in fact have essentially gone extinct as a group before the other victims of the more general Cretaceous-Tertiary mass extinction.

Mammals

The earliest fossil mammals are early Mesozoic, the exact age being dependent upon which fossils one accepts as meeting the definition of ‘mammal.’ Conventionally, mammals are recognised by their jaw morphology: how the jaw articulates with the skull and incorporation of two small bones into the inner ear [® sidebar]. The Late Triassic morganucodontids exhibit an intermediate jaw morphology, neither completely reptilian nor yet fully mammalian. However, they are the oldest and most primitive of the triconodontans so are sometimes (e.g. Rich et al. 1996, p. 519) regarded as the first mammals.

Early mammals had a Pangean distribution, including Europe, Asia, Africa, Australia and the Americas. Mammals were presumeably abundant in the Mesozoic, though their fossil record is poor, probably due to their small size which makes the fossils fragile and difficult to find.

“The Jurassic period is an important stage in early mammalian evolution, as it saw the first diversification of this group, leading to the stem lineages of monotremes and modern therian mammals. However, the fossil record of Jurassic mammals is extremely poor, particularly in the southern continents. Jurassic mammals from Gondwanaland are so far only known from Tanzania and Madagascar, and from trackway evidence from Argentina” (Rauhut et al. 2002, p. 165).

Throughout the early Mesozoic they remained small, becoming more abundant, larger, and more diverse in the Cretaceous, which may have been a time of explosive radiation of Tribosphenida – early relatives of marsupials and placentals (Rougier 2002).

In reptiles – including the mammalian ancestors – the jaw joint is hinged on two small bones; one (the quadrate) linked to the squamosal bone of the skull and the other (the articular) to the lower jaw itself (the dentary). In true mammals, the dentary is hinged directly to the squamosal; the quadrate and articular bones are incorporated into the mammals inner ear, becoming the incus and malleus respectively.

The Late Triassic morganucodontids had a jaw in which the dentary articulated with the squamosal but which still included articular and quadrate bones; these had not yet evolved to form the malleus and incus of the true mammalian inner ear.

Plant Groups

Perhaps the most significant evolutionary event of the middle Cretaceous was the great proliferation of angiosperms – the flowering plants. However, the angiosperms most probably arose from the Gnetales or possibly the Bennettitales (Willis & McElwain 2002, p. 184) earlier: perhaps as early as the Triassic or even the late Carboniferous (Qui et al. 1999).

Evidence supporting earlier dates is mainly provided by calibrated genetic divergence studies, though fossil angiosperm-like pollen and leaves have been found dating back to the late Triassic. Several form-species of Crinopolles-type pollen possessing a tectate wall have been described, dating to perhaps 220 Ma. The oldest leaves are somewhat younger, perhaps 210 Ma, and include the problematic taxa Furcula and Sanmiguelia.

Genetic evidence (Zanis et al. 2002) strongly suggests that the most ‘primitive’ (basal) living angiosperm is a little known shrub called Amborella trichopoda. Amborella is a small shrub with tiny greenish-yellow flowers and red fruit, native to the South Pacific island of New Caledonia. The Nymphaeales (waterlilies and their relatives) are also contenders for the distinction of being the most basal living angiosperms (read more).

Perhaps the most basal fossil group yet to be well-delineated within the angiosperm clade is the Archaefructaceae, a family of herbaceous aquatic plants recovered from the Lower Cretaceous or possibly uppermost Jurassic Yixian Formation of western Liaoning, China. These plants had reproductive axes that lacked petals and sepals, and bore stamens in pairs below conduplicate (sharply folded together lengthwise) carpels. One combined morphological and molecular “total evidence” analysis places the Archaefructaceae as a sister group to all extant angiosperms, including Amborella and the Nymphaeales (Sun et al. 2002, p. 900).

“Until the 1970s not a great deal was known about fossil flowers. Since then our knowledge has grown explosively. For example, the mid-Cretaceous Archaeanthus, from Russell, Kansas, now one of the best-known early flowers, has been the subject of extensive research. The flower is borne terminally on a long axis, and the seeds can be macerated out. The stamens and tepals are known from scars, resin bodies are scattered in the fruit and tepals, and all these features, together with the morphology, indicate an evident relation to the extant Magnoliacea.

Amborella trichopoda (28412


















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Fig. 9: Amborella trichopoda. [Photograph by Tim Stephens, courtesy of the Arboretum of the University of California, Santa Cruz.]

“Compression flowers in general, however, are rare, and it is usually only the large ones which are seen. Some years ago, sieving techniques used for Tertiary sediments were applied to the Cretaceous, and yielded a previously unimagined diversity of Cretaceous angiosperm flowers, from sites in North America, Sweden, Portugal, Kazachstan, and Japan. It is now becoming clear how lineages are related. The earliest pollen is 135 million years old, and many basal eudicot lineages were fully established by about 110 Ma. Insect pollination is overwhelmingly supported by the evidence, and was probably important for enhancing speciation rates. Once started, the radiation of angiosperms, especially in low latitudes, kept rising, and shows no sign of levelling off in the Tertiary” (Clarkson 1999, p. 53).
The enormous radiation of angiosperms has largely occurred since the mid-Creataceous, coevolving with a similar radiation of insects. Today the angiosperms comprise some 270,000 described species, placed in about 380 families and 83 orders (Mayr 2001, p. 64).

End of the Era

Surely the most widely reported – and the most widely misreported – of all extinction events is that which brought the age of the dinosaurs to an end at the close of the Mesozoic: the “KT” extinction.

Almost all of the popular and “lightly technical” literature published within the past couple of decades is filled with comet or meteorite impact theories, all ultimately traceable to the father and son team of Luis and Walter Alvarez. Indeed, there is a good body of evidence to support the contention that a large extraterrestrial impact did occur at the very end of the Cretaceous.

However, the KT extinction is only one of several such mass extinctions in the Phanerozoic, and by no means the largest. There is no evidence of impacts associated with the other mass extinction events, and very few people try to maintain that argument today. In fact, flood volcanism is the only mechanism which really matches the data. “[L]arge continental flood-basalt volcanic events exhibit a near-perfect stage-level association with marked increases in Mesozoic and Cenozoic extinction intensity. Ten out of the 11 major flood-basalt eruption events of the last 250 million years occur during a stage that contains a local extinction peak (i.e. mass extinction). It is presently unknown whether this association is scale-dependent or extends to smaller events as well” (MacLeod 2002).

"Everyone has a favourite theory for major extinctions, all united by the common theme of attributing dominant importance to physical factors, and playing down the importance of normal biological mechanisms. Our own work strongly favours the diversification of both mammals and birds at least 30 million years before the extinction of dinosaurs – there must be ecological consequences for small dinosaurs from this early diversification" (Penny 2002).

If an impact was the sole cause of the KT extinctions, then that would make the KT different from the several other mass extinctions known, which is not a parsimonious conclusion. However, if an impact did occur at about that time, which seems likely, no doubt it would have added further stress to the already failing ecosystems, and possibly accelerated or even heightened the extinction event in some places.

(Read more about the Cretaceous-Tertiary boundary.)

Post-Mesozoic Mammalian Evolution

Diversity

Both abundance and diversity of mammals were reduced severely by the Cretaceous-Tertiary mass extinction event (e.g. in the Hell Creek assemblage, only one out of 28 mammal species survived; Ward 2000, p. 173) with fewer taxa known from the Paleocene than from the Cretaceous.
However, the diversity and range of mammals increased greatly after the Paleocene/Eocene boundary (about 55 million years ago), and new groups appeared on continents throughout the Northern Hemisphere. On the basis of primarily phylogenetic analyses, Asia has been suggested as a likely center of origin (Bowen et al. 2002, p. 2028).

The greatest diversity appears to have been achieved in the Miocene, with a subsequent reduction through to the present day.

The Primates

The primate record, generally, and the human record in particular, is very incomplete. The closest living relatives of primates may be the Dermoptera (colugos; suggested by genetic studies) or the clade of Dermoptera and Chiroptera (bats; suggested by morphology). Any common ancestor to the three groups must have lived in the Cretaceous; probable primate ancestors – genera such as Purgatorius, Plesiadapis and Phenacolemur – date from the earliest Tertiary, approximately 65 Ma. The oldest known true primates occur about 55 Ma (near the Paleocene/Eocene boundary).

===

Fossils of bushbabies and lorises reported from deposits of the Fayum Depression in Egypt extend the known record for this group of primates from 20 million years to approximately 40 million years ago.

ERIK R. SEIFFERT*, ELWYN L. SIMONS* & YOUSRY ATTIA

Fossil evidence for an ancient divergence of lorises and galagos

Nature 422, 421 - 424 (2003); doi:10.1038/nature01489

===

The first recognisable apes had evolved by about 20 Ma ago with the appearance of Dryopithecus (formerly Proconsul) africanus, which may have been ancestral to humans. A great variety of forms appeared shortly after Dryopithecus, particularly in Africa, but the fossil record becomes more than usually poor between about 14 and 4 Ma ago, and little is known from this period. Molecular evidence suggests that it was during this period, between 5 and 10 Ma ago, that the evolutionary line of humans diverged from that of the other apes.

“A profound faunal reorganization occurred near the Paleocene/Eocene boundary, when several groups of mammals abruptly appeared on the Holarctic continents. To test the hypothesis that this event featured the dispersal of groups from Asia to North America and Europe, we used isotope stratigraphy, magnetostratigraphy, and quantitative biochronology to constrain the relative age of important Asian faunas. The extinct family Hyaenodontidae appeared in Asia before it did so in North America, and the modern orders Primates, Artiodactyla, and Perissodactyla first appeared in Asia at or before the Paleocene/Eocene boundary. These results are consistent with Asia being a center for early mammalian origination” (Bowen et al. 2002, p. 2028).
The recently described, 6-7 Ma Sahelanthropus tchadensis discovered at Toros-Menalla in Chad, is the oldest plausible human ancestor known to date. Not much younger, ~6 Ma, is Orrorin tugensis, discovered at Lukeino in Kenya. Together, the two fossil discoveries hint at a diverse and perhaps geographically widespread homonid ancestry, and an older divergence between men and apes than is indicated by most molecular studies. For the present, there is insufficient evidence to be sure.

The earliest fully bipedal human ancestor known is the 4 Ma old Australopithecus afarensis, first recognised from the famous fossil known as Lucy. Subsequently, two main lines of pre-human evolution diverged: the australopithecines, which eventually became extinct, and those given the genus name Homo, beginning with an unnamed ~2.5 Ma old fossil, rapidly succeeded by Homo rudolfensis. Two further species, Homo habilis and H. ergaster overlap through much of the interval 2 to 1.5 Ma; the latter seems more likely to have been the direct human ancestor. Homo erectus is the only recognised representative of the genus between about 1.2 and 0.7 Ma; some interpretations place it on the direct ancestral line to modern humans, others consider H. ergaster to have been directly ancestral to H. heidelbergensis, which first appears about 0.6 Ma, and from there to modern H. sapiens. However, if H. erectus is not directly ancestral to H. heidelbergensis and modern man, we are left with a ~900,000 year gap in which the true intermediaries are unknown.

Anatomically modern H. sapiens first appears close to 200,000 years ago. The oldest well-dated fossils are the Omo I and Omo II fossils from Kibish, Ethiopia (McDougall et al. 2005).

lrgWood2002Fig2.jpg


















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Fig. 10: Reproduction of fig. 2 from Wood 2002, showing the known fossil record of homonids. The asterisks indicate discoveries made since 1990 or so.

Conclusion

Although it is usually referred to as the “theory of evolution,” for all intents and purposes, evolution is a fact. It is the central fact of all biology. I won’t bother to rail against the non-sensical arguments of the creation “science” brigade here, partly because you just can’t argue with blind faith, particularly when it is strengthened by poor education and misinformed by self-serving spiritual leaders, but mostly just because it isn’t worth it. If there is no room in somebody’s world view for the plain evidence of the fossil record, it’s their loss, not ours.

(Interestingly, that most conservative of institutions, the Catholic Church, has reconciled itself to the fact of evolution. In his October 1996 address to the Pontifical Academy of Sciences, Pope John-Paul II argued for an ‘ontological discontinuity’ when God introduced a soul into ancestral apes, thus making them men.)

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