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Comment to the reader.
The previous entry described the first two billion years of life on Earth — a long, almost entirely microscopic stretch in which the chemistry of cells was worked out, the atmosphere was remade, and the cooperative architecture of multicellular bodies was assembled in the oceans. By its end, eukaryotic cells were established and several lineages had begun to live in cooperative bodies of many cells working as one. What the planet still lacked, at the close of that entry, was animals in the modern sense, plants of the kind that build forests, and any biology at all on the dry continents above the tideline.
This entry covers the period during which all of that arrived. It begins in the deep freeze of the Cryogenian, runs through the Cambrian and the long Palaeozoic afterwards, and ends with the appearance of the egg that finally let the descendants of fish reproduce out of sight of standing water. It is the period during which life moved, decisively and visibly, from being a layer of slime and fronds in the sea to being a presence on the dry surface of the Earth.
I write in the spring of 2026 of the Common Era. The dates given here are 2026 best estimates and will continue to move as dating methods improve and new fossils are described. Where a benchmark has shifted recently in the working literature, or remains genuinely contested, I have flagged it in the prose.
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How did life become visible?
For the first three and a half billion years of its existence on Earth, life was almost entirely microscopic. There were stromatolite mats, and toward the end there were filamentous algae and a handful of decimetre-scale aggregates, but to a casual observer walking along the shoreline of any continent in 2026 these would not register as life at all. The sea was busy with cells, but the world above the tideline was bare rock. By the time the period covered in this entry ends, that situation has been transformed beyond recognition. There are animals with eyes and guts and limbs. There are forests. There are creatures that lay eggs in dry soil and walk away. The change took roughly four hundred million years, and the events that drove it are mostly preserved well enough in the rock record that we can describe them in some detail.
This is the story of how life on Earth left the microscope and stepped onto the land.
The transition begins, oddly, with the planet freezing.
At intervals during the period now called the Cryogenian — running from roughly 720 to 635 million years ago — Earth fell into the most severe ice ages in its geological record. Glacial sediments of this age have been recovered on every modern continent, including localities that were unambiguously near the equator at the time of deposition. The simplest interpretation, advanced in detail in the late 1990s and progressively refined since, is that ice sheets reached all the way from the poles to the tropics, and that the surface of the oceans froze across most or all of the planet. The hypothesis is now widely referred to as Snowball Earth [1].
There were two principal episodes. The Sturtian glaciation, the older and longer of the two, began at around 717 million years ago and persisted for some 57 million years before retreating at around 660 million years ago. The Marinoan glaciation came roughly 15 million years later and ended at around 635 million years ago, with recent radiometric work constraining its fully-glaciated phase to perhaps four million years [2]. Both episodes were accompanied by the kind of carbon-isotope excursions in ancient sediments that are characteristic of severe and prolonged disruptions of the global carbon cycle.
The mechanism that triggered them is not yet completely understood. The leading account ties the onset of the Sturtian event to the break-up of the supercontinent Rodinia and a corresponding drop in atmospheric carbon dioxide as freshly exposed silicate rock weathered and consumed CO₂ from the air. Once enough ice had formed at high latitudes, the increased reflectance of the Earth's surface fed a runaway cooling: more ice meant more reflected sunlight meant more cooling meant more ice. The runaway eventually stopped only when ice covered enough of the surface that very little volcanic CO₂ was being weathered out of the atmosphere; over millions of years, volcanic outgassing built atmospheric CO₂ back up to the point where the greenhouse effect overwhelmed the planet's reflectance and the ice retreated, abruptly, into a hot post-glacial world [1].
What matters for this entry is not the climatology but the timing. The Cryogenian glaciations bracket the period immediately preceding the appearance of the first macroscopic animals. Whether the freezing acted as a filter that selected for new biological strategies, or supplied chemical conditions favourable to larger bodies (oxygenation pulses, nutrient flushes from glacial weathering have both been proposed), or simply preceded the new biology by coincidence, is not yet settled. But the next chapter of life follows immediately on the heels of the last great snowball.
The retreat of the Marinoan ice sheets opens the Ediacaran period, running from 635 to 538.8 million years ago. Within this interval, for the first time in Earth's history, there are organisms in the rock record large enough that an unaided human eye would notice them.
The earliest of these are preserved as impressions on ancient sea-floor sediments at Mistaken Point in Newfoundland, in deep-water deposits dating to roughly 574 million years ago. The forms are unlike anything alive today. Many are rangeomorphs — frond-shaped organisms whose surface is covered with a self-similar fractal branching pattern that repeats across at least four levels of scale. The largest examples reached up to about two metres tall. They were anchored to the sea floor by a basal disc and presumably absorbed nutrients directly from seawater, having no mouth, no gut, and no detectable internal organs of any kind. Charnia masoni, first described from rocks of similar age in Charnwood Forest in England, is the type specimen of the group [3].
Slightly younger Ediacaran assemblages, dated between roughly 560 and 550 million years ago and preserved in localities including the White Sea region of Russia, the Flinders Ranges of Australia, and Namibia, contain a wider range of forms. Among these are Dickinsonia, an oval, ribbed, mat-like organism reaching as much as a metre in length, and Kimberella, a smaller bilaterally symmetrical creature that left scrape marks on the sea floor consistent with a muscular foot used to graze the microbial mats it lived among. There are also tubular and stalked forms, frond-like forms, and a small number of organisms that may anticipate later sponge or cnidarian body plans.
The biological affinities of the Ediacaran organisms remain genuinely contested in 2026. Charnia and the rangeomorphs do not match any modern phylum cleanly and may belong to an entirely extinct branch of multicellular life. Dickinsonia yielded animal-specific cholesteroid biomarkers in a 2018 study of preserved organic residues, and on that basis is now widely treated as an early animal, although what kind of animal remains unclear [4]. Kimberella is the most plausible early bilaterian — an organism with a clearly defined front and back, top and bottom — and is the best candidate for an Ediacaran member of a lineage with descendants alive today. The most cautious framing is that the Ediacaran biota records the first body-plan-scale experiment in macroscopic multicellular life, that some of its members were animals, and that some were almost certainly not.
By the close of the period, around 540 million years ago, most of the distinctive Ediacaran forms had disappeared from the fossil record. Whether they were actively eliminated by the appearance of the new mobile, predatory animals that came next, or whether they declined for environmental reasons that the rocks have not yet revealed, is one of the active questions of early-animal palaeontology.