Evolutionary History of the Earth

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This is a simple synopsis of the major events in the history of life, focusing on the history of animals. This assumes little prerequisite knowledge except for the Geologic Time Scale, around which it is organized.

Hadean Eon - 4.56 - 4.03 Ga (Note Ga = billion years ago).

  • Formation of Earth through accretion of planetesimals.
  • Accumulated heat of bombardment initially melts Earth's surface.
  • Differentiation of core, mantle, crust.
  • Formation of Moon from debris torn off of Earth by impact with Mars-sized impactor.
  • Early atmosphere mostly CO2, so oceans highly acidic.
  • No life.
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    Archean Eon - 4.03 - 2.5 ga.

  • 3.8 Ga: Oldest clastic sedimentary rocks in a continental environment. Liquid water was flowing!
  • 3.7 Ga: Oldest strong evidence of life is fractionated carbon in deep sea sediments preserved in Greenland.
  • 3.4 Ga: Oldest strong body fossils - Fig Tree Cherts

  • Photosynthesis: For a while, organisms got away with eating the organic materials that were floating around in the ocean. As these started to get scarce, one group, the cyanobacteria, came up with a new method of capturing energy from the environment - Photosynthesis,

    6 CO2 + 6 H2O + energy (sunlight)---> C6H12O6+ 6 O2

    Note that oxygen is a product.

    We can't tell from looking at microscopic fossils which were photosynthesizers, but photosynthesis had momentous consequences for the rock record. As oxygen is released into ocean water, it quickly reacts with ions such as iron, that act as oxygen sinks. ~3.0 Ga: Banded Iron formations (BIFs), marine sedimentary rocks with alternation of gray and rust red bands of hematite (Fe2O3) begin to appear, indicating that photosynthesizers were at work.

  • The age of slime: At roughly the same time we pick up Stromatolites, laminated bacterial mats. These were very common for until the end of the Proterozoic, when critters appeared that could eat them. Occur today only in unusual environments that protect them from grazers, like hypersaline Shark Bay in Australia.
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    Proterozoic Eon - 2.5 - 0.542 Ga.

  • 1.8 Ga: Terrestrial red bed deposits begin to appear, indicating that the oceanic oxygen-sinks had become saturated and free oxygen was now building up in the atmosphere and reacting with terrestrial sediments. Confirming this, BIFs quit forming in the oceans.
  • Ozone: As it accumulated, free oxygen in upper atmosphere recombined to form ozone layer (O3), allowing life to colonize surface waters.
  • 2.7 Ga: Eukaryotes appear, probably formed by endosymbiosis (prokaryote-grade organisms joining together to form a more complex cell). Oldest biochemical markers of eukaryotes - steranes trapped in sediments.
  • 2.2 Ga: Oldest eukaryotic body fossils.
  • 1.7 Ga: Acritarchs, the resting stage of some eukaryotic organism. Common and diverse microfossils, reaching peak diversity around 850 Ma (million years ago) and lasting into the Phanerozoic.
  • 830 - 600 Ma: Snowball earth episode - a series of drastic ice ages (the most severe on record). Although controversial, some evidence indicates glacial and oceanic pack ice at the equator. This seems to have exerted a strong selective pressure, because soon afterward, we witness the independent derivation of multicellularity in:
    • Animals
    • "Green algae"
    • Fungi
    • Red Algae
    • Brown Algae

  • We now switch to a smaller time scale


    Ediacaran Period - 630 - 542 Ma. - The last period of the Proterozoic

  • Some controversial trace fossils from c. 800 Ma (some date to 1100 Ma, others think only 570 Ma)
  • Definite animal-generated trace fossils by 570 Ma

    The Ediacaran fauna, preserved only as impressions in sediment, records the first animal fauna:

    • Definite early cnidarians, large polyps with their butts in the sand
    • "Vendobionts"
      • A unique (and extinct) radiation of multicellular forms with a quilt-like body construction: see here and here.
      • May have fed by with the aid of photosynthesizing symbiots (there is no evidence for mouths or anus in these creatures)
    • Kimberella: apparently an early mollusk or mollusk-relative; known from impressions of its foot and probable grazing marks
    • Spriggina: Considered by some to be an attached frond-like organism, by others to be an active relative or precursor to arthropods.
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    Cambrian and Ordovician Periods 542 Ma. - Beginning the Paleozoic Era of the Phanerozoic Eon with explosive diversification

    Cambrian 542 - 488.3 Ma
    • 542 Ma: First appearance of burrowing trace fossil Trichophycus pedum. Signaled big ecological change from the Ediacaran, in which sediment was undisturbed by burrowers, allowing algal mats to grow abundantly. The Phanerozoic, in contrast, is an age in which animals churn and disturb ocean sediments.
    • 521 Ma: The Cambrian Explosion Historically, the first appearance of macroscopic shelly fossils. Enigmatic because many lineages learned the trick of secreting hard skeletons at roughly the same time. Could be due to the rise of predators capable of devouring an animal, or to changes in sea-water chemistry making it biochemically easier.
    • Based on statistical work by Jack Sepkoski, marine invertebrate communities are often broken down into three separate “evolutionary faunas”:
      • The Cambrian fauna (or Trilobite fauna): trilobites, archaeocyathids, etc.
      • The Paleozoic fauna (or Brachiopod fauna): brachiopods, bryozoans, stromatoporoids, cephalopods, crinoids and blastoids, starfish, graptolites
      • The Modern fauna (or Bivalve-Gastropod fauna): bivalves, gastropods, vertebrates, sea urchins, crustaceans, etc.
      All three categories exist in the Cambrian, and persist until the present (even if some component members have died off). However, these "packages" of distantly related groups tend to be common at the same time, or rare at the same time.

    • Almost no large animals, and very few predators (largest animal is only large predator, 1 m long anomalocaridids)
    • Archaeocyathids: Probably armored sponges. The first reef-formers.
    • Trilobites: Terrifically common arthropods of the early Paleozoic.
    • Cambrian vertebrates known from bony plates and impressions of lamprey-like forms.
    Ordovician 488.3 - 443.7 Ma


    The late Early Paleozoic - Silurian (443.7 - 416 Ma) and Devonian (416 - 359.2 Ma) Periods

  • Great diversification of jawless and jawed vertebrates
  • Invasion of the land by:
      • Terrestrial "green algae"
      • First evidence of possible terrestrial plants from vascular tissue and spores of the Ordovician
      • Early vascular plants of the Silurian: use tubes in body transport nutrients & water upwards and manufactured food throughout body L
      • A famous early land plant is Cooksonia, which has spore-bearing organs, vascular tissue, waxy surface, stomata, but NO leaves. Rarely over 10 cm. tall.
      • By the end of the Devonian, the first tree-sized plants were spreading - the first forests
      . Where they spread, they created habitat for......
    • Arthropods, including:
      • Myriapoda (centipedes and millipedes)
      • Arachnida (scorpions, etc.)
      • Hexapoda (insects and their relatives)
    • Toward the end of the Devonian, jawed vertebrates tentatively join the land-rush, through forms like Acanthostega. These are close to the ancestry of proper tetrapods, including living amphibians and amniotes.
  • Near the end of the Devonian, a major mass extinction event greatly reduces some groups, including trilobites and primitive jawed vertebrates.
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    The Late Paleozoic - Mississippian (359.2 - 318.1 Ma), Pennsylvanian (318.1 - 299 Ma), and Permian (299 - 251 Ma) Periods

  • After the Devonian extinctions knocked out most odd jawless vertebrates and primitive gnathostomes, the oceanic vertebrate fauna took on a more modern look, with diverse sharks, ray-finned fish, and the occasional lobe-fin.
  • First seed plants appear
  • Forests proliferate in moist tropical environments, giving rise to massive coal deposits. Hence...
  • In Europe, geologists don't acknowledge distinct Mississippian and Pennsylvanian Periods, but group them as the Carboniferous.
  • These are inhabited by ever more diverse arthropods, including very large winged insects, and tetrapods close to the ancestry of amphibians. Creatures that depended on bodies of water for reproduction.
  • During the Pennsylvanian, they were joined by primitive amniotes, including early representatives of:
    • Reptilia - the monophyletic group containing modern turtles, lizards, crocs, and birds.
    • Synapsida - the dominant land group of the Late Paleozoic, the monophyletic group containing modern mammals. Primitive Synapsids formed the worlds first large terrestrial carnivore and herbivore fauna.
  • Synapsids diversified through the Permian.
  • Reptiles including primitive archosaurs (related to living crocs and birds) were a minor component of the fauna...waiting......
  • At the end of the Permian, Earth experienced the most extreme mass extinction in its history. Over 90% of marine species and 70% of terrestrial species extinguished. Planet come frighteningly close to being sterilized.
    • In the oceans, trilobites, sea-scorpions, horn and tabulate corals, many stalked echinoderms extinguished. Crinoids, brachiopods reduced to minor faunal components, major turnover of vertebrates.
    • On land, synapsid diversity greatly reduced. Remaining synapsids typically were smaller as well.
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    The Early Mesozoic - Triassic (251 - 199.6 Ma), Jurassic (199.6 - 145.5 Ma) Periods

  • The Permian extinction changed the face of marine invertebrate faunas in much the way that the Devonina extinction did for vertebrates. With trilobites, sea-scorpions, and blastoids gone, and crinoids and brachiopods smacked down to obscurity, crustaceans, cephalopods, clams, and sea-urchins diversified, giving the oceans a more modern look. But they were joined by....
  • Marine reptiles - the first amniotes that re-invaded the oceans.
  • On land, the major synapsids were now possum sized. Only the herbivorous dicynodonts retained their Paleozoic proportions. Reptiles moved into the dominant land predator and herbivore positions, with archosaurs and their close relatives taking the lead. During the Triassic, these included: As of the Middle Triassic, it was not clear who would end up with permanent ownership of these ecological niches. That got sorted out by a major mass extinction event near the end of the Triassic. Rauisuchians, rhynchosaurs, phytosaurs, and aetosaurs were snuffed. Dinosaurs, pterosaurs, crocodylomorphs, and turtles moved to the next square.
  • By the Jurassic, those synapsids that survived were miniaturized and were close to the ancestry of proper mammals.
  • The Jurassic saw dinosaurs assume dominant positions as both predators and herbivores. Among them, small bipedal carnivores evolved feathers as an integument and becan to apply these to problems of locomotion. Late Jurassic dinos like Archaeopteryx were on the verge of joining pterosaurs in the air.
  • With their phytosaur competitors eliminated, crocs took over the fresh water aquatic niche. (Wish occasional excursions into the oceans.)
  • Jurassic dinosaurs shared their world with newcomers - proper lizards and rhynchocephalians.
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    The Late Mesozoic - Cretaceous (145.5 - 65.5 Ma) Period

  • The Cretaceous represents the full flowering of Mesozoic diversity.
  • In the oceans:
    • The first squids and octopuses joined shelled ammonoids and belemnoids as free-swimming cephalopods.
    • Mosasaurs - true lizards, joined other marine amniotes as major predators. They competed with...
    • Essentially modern-looking lamnoid sharks.
    • Clam reefs: One odd thing. Since the Permian extinction the oceans had lacked major reefs. These returned in the Cretaceous, but composed of rudistid clams.
  • On land:
    • Flowering plants spread and diversify
    • Non-avian dinosaurs reached their maximum diversity.
    • Crocodylomorphs gave rise to the first proper crocodiles, alligators, and gavials.
    • True mammals, identifiable as placentals, marsupials, and monotremes; hid in the undergrowth, none larger than a house cat.
    • A diverse array of bird-like dinosaurs competed with the largest pterosaurs on record.
  • Just when everything looked good, another mass extinction. This is the best studied event of its sort and seems to involve the effects of:
    • a major asteroid impact (recorded in the Chicxulub structure in Yucatan)
    • large scale volcanism (the Deccan Traps in India)
    On land, this event completely extinguished pterosaurs and all dinosaurs but a handfull of birds. Crocs, mammals, and turtles came through in good shape. In the oceans, ammonoids, many types of microscopic plankton, and all marine reptiles were extinguished.
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    The Cenozoic Era - Paleogene (65.5 - 23 Ma) and Neogene (23 Ma - Present) Periods

  • Released from their competition with dinosaurs, modern mammals radiate into a variety of forms and niches, ecologically replacing them, with some competition from birds and crocs.
  • Mammals develop flight as bats.
  • Mammals invade the oceans as cetaceans, and later sea-cows and pininpeds.
  • The worldwide proliferation of grasses in the Paleogene stimulated the evolution of grazers.
  • During the Neogene, global cooling, combines with grasslands to create a physical environment very like what we see today.
  • The Pleistocene epoch (the last 2 million years) witnesses a major ice-age. That, combined with the spread of humans, caused a final major (and ongoing) extinction event.