Geologic Time Scale Spiral Ics Colours (Light) Path Text
9.1 From the origin of the Earth to eukaryotic cells #
9.1.1 The age of the Earth, and early cells #
The Earth is about 4.54 billion years old, from isotopes in meteorites that formed with the solar system and from the oldest terrestrial minerals. The Hadean, before about 4.0 billion years ago, has no uncontested fossil record. The Archaean runs from about 4.0 to about 2.5 billion years ago, and the Proterozoic from then until the Cambrian at about 540 million years ago. These are rounded boundaries of the geological timescale, not dates of evolutionary events, except where an event is used to define a boundary.
Cellular life is Archaean. The cautious statement is that widely accepted evidence of cells is more than about 3.4 billion years old. Older candidates, both structural and chemical, are still argued over. Naming one specimen as the first cell would pretend to a certainty the rocks do not have. The secure order is the Earth, then cellular life well within the Archaean, then a long Proterozoic, then animals with hard parts near the end of it.
Cyanobacterial oxygenic photosynthesis drove the Great Oxidation Event about 2.4 billion years ago. Oxygen stayed far below modern levels for a long time afterwards. Later rises, and any link to animals, are more disputed. Oxygen made an aerobic biosphere possible. It is not, by itself, an explanation of the Cambrian diversification.
Eukaryotes are later. Fossils widely treated as eukaryotic are Proterozoic, more than about 1.5 billion years old, and both older claims and the age of the last eukaryotic common ancestor are uncertain by large margins. The cell-biological definition — nucleus, cytoskeleton, mitochondria or organelles derived from them — is Cell Biology. The evolutionary fact used here is that eukaryotes are a clade with a chimeric history.
9.1.2 Endosymbiosis #
Endosymbiosis is the origin of an organelle from a free-living cell that took up residence inside another. Mitochondria descend from alphaproteobacteria. Chloroplasts descend from cyanobacteria. Genomes, bacterial ribosomes, double membranes, and sequence affinity are the evidence reviewed in Cell Biology.
Mitochondria, or organelles derived from them, occur across eukaryotes. The endosymbiosis happened on the stem of the last eukaryotic common ancestor, or so close to it that no living eukaryote retains a pre-mitochondrial condition. Many mitochondrial genes have since moved to the nucleus. A eukaryote that has lost the mitochondrial genome has not thereby skipped the endosymbiosis.
Primary chloroplasts were acquired once, in the ancestor of glaucophytes, red algae, and green plants, including land plants. Euglenids, dinoflagellates, diatoms, and brown algae gained plastids by secondary endosymbiosis, engulfing a eukaryote that already had one. Eukaryotic photosynthesis is one cyanobacterial symbiosis, spread by inheritance and by further symbioses. Ecological “plants”, meaning organisms that photosynthesise with a plastid, are not one clade. The chemistry of the light reactions belongs to Cell Biology and plant physiology.
9.2 Animals and the land #
9.2.1 The Cambrian diversification #
The Cambrian runs from about 540 to about 485 million years ago. Early in it, the fossil record acquires most of the major animal body plans that have hard parts, plus soft-bodied forms in exceptional deposits. Chengjiang is about 518 million years old, the Burgess Shale about 508. This is the Cambrian diversification. “Explosion” describes a geological rate. It does not mean one moment, and it does not mean that evolution worked differently for twenty million years.
What is real is an expansion of preservable animal bodies, bilaterian organisation, and hard parts, fast relative to the later Palaeozoic average. What is not established is that every modern phylum originated in that window from an undifferentiated ancestor, or that each Burgess oddity founded a living phylum. Some fossils are stem members of living groups. Some are still hard to place. The Ediacaran biota, from about 575 million years ago to near the Cambrian boundary, already shows macroscopic organisms, many of uncertain affinity. The Cambrian is not the first large life. Small shelly fossils and traces precede the famous soft-bodied faunas.
Soft bodies rot, so the origin of mineralised skeletons makes a diversification look sharper than the origin of the lineages. Molecular clocks generally put divergences among animal phyla earlier, in the late Proterozoic, with large error. Animals did not begin at the base of the Cambrian from nothing. The Cambrian record does show a real, geologically rapid increase in animal body plans and in hard parts. It is a radiation, not a ladder from jellyfish to fish.
9.2.2 Colonisation of land #
Land was occupied in an order the fossils support at period resolution: plants in the Ordovician, arthropods in the Silurian, vertebrates in the Devonian, with the qualification below.
| Colonists | Period | What the fossils show |
|---|---|---|
| Land plants | Ordovician spores, about 470 million years ago; forests in the Devonian | A lineage on land, not immediately a woodland |
| Arthropods | Silurian into the Devonian | Trackways and body fossils on land before vertebrate limbs |
| Vertebrates | Limbs in the Late Devonian, about 375–360 million years ago; amniotic egg in the Carboniferous | Limbs in animals that were still largely aquatic |
Cooksonia and similar upright stems are Silurian. Forests, and the soils they build, are Devonian. Arthropods, including early relatives of insects and arachnids, were on land before tetrapods. Tiktaalik and other stem tetrapods are aquatic animals with gills as well as limbs. “Vertebrates colonised the land in the Devonian” is right as an order of magnitude and wrong as a picture of an animal walking ashore and staying. A life cycle freed from laying eggs in water is the amniotic egg, in the Carboniferous. The move onto land is a sequence of partial commitments spread over more than a hundred million years.
Rounded boundaries, for orientation and not as event dates: Cambrian from about 540 million years ago, Ordovician 485, Silurian 444, Devonian 419, Carboniferous 359, Permian 299, Triassic 252, Jurassic 201, Cretaceous 145, Cenozoic 66. Hominins occupy the last few million years of that last interval.
9.3 Mass extinction #
9.3.1 The big five #
A mass extinction is a geologically short interval in which extinction rises far above background across many groups and the ecological cast is rearranged. Five Phanerozoic events are grouped as the big five. Species-loss figures are conventional approximations, not censuses. Genera and families give lower percentages than species, clades suffer differently, and the record is incomplete. The numbers below are the standard “about” figures. They should not be repeated as if someone had counted every species on the last day of the Permian.
| Event | About when | Approximate species loss often quoted | Cause, and how firm it is |
|---|---|---|---|
| End-Ordovician | 444 million years ago | About 85% of marine species | Glaciation and sea-level change; not as singly established as an impact |
| Late Devonian | A series, ending about 359 million years ago | About 70–75% | No single cause is established |
| End-Permian | 252 million years ago | About 90% or more of marine species | Siberian Traps volcanism is the leading explanation |
| End-Triassic | 201 million years ago | About 75% | Central Atlantic volcanism is the leading candidate, not a closed case |
| End-Cretaceous | 66 million years ago | About 75% | The Chicxulub impact is established |
The end-Cretaceous cause is established. An iridium anomaly, shocked minerals, and the Chicxulub crater dated to the boundary are the impact evidence. Non-avian dinosaurs, pterosaurs, ammonites, and much marine plankton do not continue. Birds, which are dinosaurs, do. Mammals do, and they diversified into space the extinction had emptied. They did not out-compete dinosaurs through the Mesozoic. Deccan volcanism was underway in the same interval and may have added stress. It is not the established cause of the boundary. The impact is.
The end-Permian is the largest. The leading explanation is the Siberian Traps, and the favoured chain runs through warming, ocean anoxia, and acidification. “Leading” means the explanation the evidence favours. It does not mean every chemical step is settled. Recovery of marine communities took far longer than the extinction. The end-Triassic has a similar candidate, the Central Atlantic Magmatic Province, and the case is less closed.
The end-Ordovician extinction is associated with Gondwanan glaciation and a fall and rise of sea level. That link is widely accepted and is not an impact-style demonstration. The Late Devonian extinctions are a series, reef life was hit hard, and no single cause is established. Mass extinctions do not replace selection, drift, or speciation. They raise the rate at which lineages end, often with an unusual selectivity, and survivorship is not a certificate of superiority in the world before the filter.
9.4 Key terms, summary, and self-check #
Key terms. Archaean; Proterozoic; Great Oxidation Event; endosymbiosis; secondary endosymbiosis; Cambrian diversification; Ediacaran; amniotic egg; mass extinction; Siberian Traps; Chicxulub.
Summary. The Earth is about 4.54 billion years old, and widely accepted cellular life is more than about 3.4 billion years old, without a named first cell. Mitochondria descend from alphaproteobacteria and primary chloroplasts from cyanobacteria; secondary endosymbiosis spread plastids further. The cell-biological evidence is taught elsewhere. The Cambrian records a real and rapid expansion of animal body plans, sharpened by hard parts, preceded by Ediacaran life, and not the origin of animals from nothing. Plants, arthropods, and vertebrates move onto land in that order, across the Ordovician, Silurian, and Devonian; Devonian tetrapods were still largely aquatic, and the amniotic egg is Carboniferous. Of the five great extinctions, the end-Cretaceous impact is established and end-Permian volcanism is the leading explanation. The familiar species-loss percentages are approximate, not censuses.
Self-check.
1. State the age of the Earth and the cautious lower bound used here for cellular life. Why is a named “first fossil” avoided?
2. Which bacterial groups gave rise to mitochondria and to primary chloroplasts? How does secondary endosymbiosis differ?
3. What is securely true of the Cambrian diversification, and what preservation bias stops it being read as the origin of animals from nothing?
4. Put land plants, terrestrial arthropods, and tetrapods in order. Why does “vertebrates walked onto land in the Devonian” overstate the fossils?
5. For which mass extinction is the cause established, and what is it? For which is volcanism the leading explanation? Why are the percentages not censuses?
