The history of life

LUCA, the last universal common ancestor, is the population of cells from which all life on Earth descends, and researchers reconstruct its characteristics by comparing millions of genes across the living world.

Alkaline hydrothermal vents are springs on the deep sea floor where seawater that has reacted with the rock comes back out warm, alkaline and rich in hydrogen, and they are one proposed setting for the origin of life.

The oldest traces claimed for life, about 3.7 billion years old, are stromatolites, layered structures built by mats of microbes, and carbon with the chemical signature of living cells, and some claims go back 3.95 billion years.

The oldest reef built by life is a platform of carbonate rock in Western Australia, about 3.43 billion years old, where microbial mats, thin sheets of microbes, raised mounds and cones.

Oxygenic photosynthesis, which takes its electrons from water and releases oxygen, arose in microbes between about 3.2 and 2.8 billion years ago, by means of a cluster of four manganese atoms in the apparatus called photosystem II.

The mitochondrion, a compartment of complex cells, began as a bacterium that came to live inside another cell about 1.9 billion years ago and transferred most of its genes to that cell's nucleus.

The eukaryotes, organisms whose cells have a nucleus enclosing their chromosomes, appeared between about 2.0 and 1.8 billion years ago, when everything alive was still a bacterium or an archaeon, two kinds of cell without a nucleus.

Protosterols, intermediate molecules in the making of the sterols of cell membranes, abound in rocks from 1.64 billion to 800 million years ago, where they are the only trace of an early biota, or set of living organisms.

Sexual reproduction, in which meiosis halves the genome of a cell and fertilisation restores it, is at least 1.2 billion years old, and the genes that run meiosis occur throughout the eukaryotes, the organisms made of complex cells.

The earliest animal was most likely a sponge, and the two fossil candidates, one from 890-million-year-old Canadian reefs and one from the end of the Ediacaran period, lie some 350 million years apart.

The Ediacara biota was a community of large soft-bodied organisms, fronds, discs and ribbed ovals, that lived in the sea between 575 and 539 million years ago and resembled no later organism.

The Cambrian explosion is the appearance of nearly all animal body plans, including those of arthropods, molluscs, annelids and the first chordates, within about twenty million years, from 540 to 518 million years ago.

The earliest known vertebrates were swimming animals a few centimetres long, with a notochord, a pair of camera eyes and zigzag muscle blocks, that lived about 518 million years ago.

Muds at Chengjiang, in the Canadian Rockies and at a few other sites preserved gills, guts, eyes and nerve cords as films of carbon for a few tens of millions of years, from about 518 million years ago.

The oldest eyes preserved in detail are fossils about 515 million years old from a shale, a rock formed from hardened mud, in South Australia, and one pair has at least sixteen thousand lenses in each eye.

The first plants to live out of water were small, flat and rootless plants of about 470 million years ago, closer to a liverwort, a low plant related to mosses, than to a plant with a stem, and are known mostly from their spores.

The end-Ordovician extinction, about 445 million years ago, is the first of the five great extinction crises and the only one caused by cold, when tropical seas cooled by about five degrees and ice sheets grew.

Tetrapods, the four-limbed vertebrates, left their oldest known trace as footprints on a tidal flat at Zachelmie, in Poland, about 390 million years ago, eighteen million years before the oldest bones of such an animal.

The seed, which appeared about 372 million years ago, freed plants from the film of water that ferns and mosses need, because pollen carries the male cell on the wind and the embryo travels sealed inside a food supply.

Synapsids, the lineage leading to mammals, split from sauropsids, the lineage leading to reptiles and birds, between 315 and 300 million years ago, when the amniotes, vertebrates independent of water, divided in two.

The end-Permian extinction is the most severe extinction on record, set off about 252 million years ago when the Siberian Traps, a vast volcanic province, erupted through coal beds and released carbon for perhaps a million years.

The ancestors of mammals became warm-blooded about 233 million years ago, in the Late Triassic period, when they raised their body temperature by five to nine degrees and began holding it constant.

The oldest animals universally accepted as dinosaurs, about 233 to 230 million years old, were small, fast bipeds the size of a large dog, found in Argentina and Brazil.

The first mammaliaforms, a group wider than mammals, were shrew-sized and probably nocturnal, between 220 and 205 million years ago, with a jaw hinged directly on the skull and molars that met precisely enough to chew.

Sauropods, long-necked herbivorous dinosaurs, reached ten times the mass of the largest land mammal through several traits, each of which made the next possible, between 190 and 145 million years ago.

Flowering plants, whose flower encloses the seed in a closed structure called the carpel and displays itself to animals, began about 140 million years ago or earlier, and the date of their origin is the longest-running argument in botany.

Bees descend from predatory wasps that began to feed their larvae on pollen instead of insects, between 123 and 100 million years ago, in western Gondwana, a former southern continent, as the flowering plants spread.

An asteroid some ten kilometres across struck a shallow sea off the Yucatán sixty-six million years ago, on a day of the northern hemisphere spring.

Placental mammals came through the extinction at the end of the Cretaceous in fewer than ten lineages, and within a few million years their descendants grew from the size of a rat to the size of a bear.

Archicebus achilles, found in China and about 55 million years old, is the oldest nearly complete primate skeleton, with the grasping hands, forward-facing eyes and nails that define the group.

Sahelanthropus tchadensis was an ape that seems to have walked upright, known from a distorted skull and a few limb bones found in Chad and about 7 million years old.

Hominins, members of the human lineage, left Africa long before our species existed, and small-bodied, small-brained hominins lived at Dmanisi, in the Caucasus, among sabre-toothed cats and giant hyenas about 1.8 million years ago.

Neanderthals were a human lineage that lived in Europe and western Asia from about 430,000 to 40,000 years ago, in small populations isolated from one another, and a sister lineage of our own, descended from the same ancestors.

The oldest fossils attributed to Homo sapiens are a skull and a jaw found with stone tools at Jebel Irhoud, in Morocco, buried about 315,000 years ago, and the face of the skull is already like ours.

People reached Sahul, the continent that then joined Australia and New Guinea, about 65,000 years ago, by crossing at least eighty kilometres of open sea with no land in sight.

Modern humans left Africa in one major expansion between 60,000 and 40,000 years ago, according to the genomes of 142 populations, and this expansion was not their first exit.

The Americas were the last habitable continent that people reached, by way of the Beringia land bridge and the Pacific coast, and human footprints at White Sands, in New Mexico, are dated to about 23,000 to 21,000 years ago.

Agriculture began about 11,500 years ago, when people sowed what they had harvested, weeded and kept herds nearby, long before the plants and animals concerned took their domestic form.

Descent with modification, the theory that Darwin published in 1859, gives every living thing a common ancestry and explains without a designer how organisms fit their environment.

Gregor Mendel counted pea plants for eight years and established in 1866 that traits do not blend in inheritance but are passed on as discrete units that separate and recombine.

Penicillin was discovered in 1928 on an uncovered culture plate, where bacteria dissolved around a spot of mould, and it took another decade, a second laboratory and a wartime industrial effort to make it a medicine.

The double helix, the structure of DNA established by three papers published side by side in Nature in April 1953, is two paired strands whose arrangement explains how the molecule is copied.

The genetic code, solved between 1961 and 1966, is the rule that assigns one amino acid, a building block of proteins, to each group of three bases, almost identically in every living thing.

The molecular clock, proposed between 1962 and 1965, is a method that dates the separation of two species by counting the differences between their molecules, on the premise that mutations accumulate at a roughly constant rate.

The archaea are microbes that Woese and Fox recognised in 1977 as a third domain of life, distinct from bacteria, by comparing one molecule present in every living thing, the 16S ribosomal RNA.

A centimetre of clay at the Cretaceous boundary, where that period's rocks end, is tens to hundreds of times richer than the rock around it in iridium, a metal scarce in the Earth's crust and common in meteorites, as reported in 1980.

The human genome, three billion letters long, was read in draft in 2001 and declared finished in 2003 with eight per cent still missing, and the last gaps were filled only in 2022.