The major transitions of life

01

From Chemistry to Heredity

Every cell alive descends from one population, called LUCA, the last universal common ancestor. Nothing of it survives, and dating it takes a detour. Certain genes were duplicated before LUCA, so both its descendant lines, bacteria and archaea, inherited two copies. Each copy recorded the split between bacteria and archaea on its own: the same event is measured twice, and the date comes out firmer. Older than LUCA, the duplication also fixes the age LUCA cannot exceed1.

That calculation places LUCA about 4.2 billion years ago, between 4.09 and 4.33, with a genome of at least 2.5 million bases, as many as a bacterium carries today, coding some 2,600 proteins. That cell needed no oxygen: it built its carbon compounds from carbon dioxide and hydrogen, and already lived among other cells1.

Almost nothing remains of what came before. Meteorites deliver amino acids ready made. Alkaline vents hold, across their mineral walls, a difference in acidity like the one every cell maintains across its membrane. And an RNA carrying the instructions while speeding up the reactions, alone, is the account that assumes the fewest parts.

02

Capturing Light, Changing the Air

Photosynthesis began without producing any oxygen. Its earliest versions used light to pull electrons from dissolved iron or sulphur compounds, and left the air untouched. Pulling them from water instead takes far more energy, and the cyanobacteria that managed it, the blue-green microbes that still form pond scum, released oxygen as waste.

The waste built up slowly at first, because dissolved iron and volcanic gases consumed it as fast as it appeared. Those consumers ran out about 2.4 billion years ago, and free oxygen entered the air. Geologists call this the Great Oxidation Event.

The rise then stopped low, and Trost and colleagues1 explain that ceiling by biology itself. Cyanobacteria draw their nitrogen from the air with an enzyme, nitrogenase, that oxygen destroys; past roughly 2 per cent oxygen in the air they would cut off their own nitrogen supply. The banded iron formations of this era, rock striped red and grey by alternating iron-rich and iron-poor layers, record that oxygen arriving: the iron left the water as it came.

03

The Complex Cell

One cell entered another and stayed. The one that took the other in belonged to the Asgard archaea, microbes without a nucleus, the closest known relatives of our own cell type. The guest was a bacterium that became the mitochondrion, the compartment in which our cells now burn their food1.

Oxygen did not found this partnership. Genomes and metabolism point to a first arrangement in which one partner produced hydrogen and the other consumed it, a trade that biologists call syntrophy, carried on where no oxygen reached. The deep ocean stayed free of oxygen for a further billion and a half years after that arrangement2.

Neither fossils nor molecular clocks tie the merger to either of the planet's two oxygenations, the one 2.4 billion years ago and the Neoproterozoic one: it falls between them2, between roughly 2.7 and 1.8 billion years ago3. Everything descended from that cell is a eukaryote, a cell that keeps its genetic material inside a nucleus. A long branch separates the first eukaryote from the common ancestor of all living ones, and no fossil intermediate sits on it3, so the order of assembly is still open.

04

Sex, and Bodies of Many Cells

Sexual reproduction runs a cycle in two steps. Two cells fuse, which doubles the number of chromosomes; a division called meiosis then halves it again, so the count returns to where it started. Nearly all eukaryotes run this cycle, and their common ancestor already did, which takes it back as far as the complex cell itself1.

What the cycle is for is still argued over. Many lineages have given up one or other of its two steps, and comparing them shows which parts selection never lets go of2: the pairing of matching chromosomes, the cell's own cutting of both DNA strands, and the repair that copies from the matching chromosome. All three mend damaged DNA. What lineages do drop lies elsewhere: the halving and the need for two parents. Hörandl and Speijer3 tie the whole cycle to reactive oxygen, the unstable molecules the young mitochondrion released, which broke DNA faster than the cell could mend it.

Bodies of many cells came later, separately and often: some forty-five independent lineages4. Only five, all eukaryotes, went on to build tissues in which cells take on different jobs5.

05

The Animal Body and Its Senses

Animals enter the fossil record first as soft impressions, then as burrows, then as shells, and at last with the full range of body plans that still exists today, the basic architectures that set a jellyfish apart from a worm or a snail.

The uranium in a Namibian rock section decays at a known rate, and geologists read from it the date of that changeover: no more than 410,000 years separate the skeletonless Ediacara fauna from a burrowing Cambrian one, between 538.99 and 538.58 million years ago1.

Burrows and trails blur that date. The range of architectures and behaviours was already wide at the very start of the Cambrian, whereas a rearrangement of ways of life arrives several million years later, one stage of the geological scale further on2. It moved the animals that strain food from the water down into the sediment, and left the sea floor feeding on what the open water produced above it.

Eyes, teeth, mineralised skeletons and backbones all enter the record across the Ediacaran and the early Cambrian. Each one raises both the price of being prey and the reward of being a predator.

06

Leaving the Water

Plants did not colonise dry land on their own. They did it with soil fungi, in an exchange where the fungus runs its threads through the ground, takes up phosphorus and nitrogen and hands them over, and the plant returns fats it has built with the energy of sunlight1.

The land plants whose lineages separated earliest from the rest all keep this arrangement, and the same genes run it in all of them, inherited from a common ancestor that therefore already practised it, some 450 million years ago, before roots existed1. Grown with these fungi rather than without them, liverworts, flat green plants of the oldest surviving group, take up more carbon and grow larger, and the gain widens under the carbon dioxide levels of the early Palaeozoic2.

Plants then built what dry land demands: tubes to carry water upward, wood to stand upright with, seeds that travel without water, and canopies dense enough to make soil beneath them. Animals came ashore separately, arthropods before vertebrates, and the first four-limbed backboned animals used their limbs in water before they used them on the ground.

07

The Egg, Warm Blood, Milk

Laid in open air, an amphibian egg dries out and its embryo has nothing to breathe: it needs a pond. The amniotic egg carries its own, sealed under the shell. Four membranes share the work: one bathes the embryo in fluid, one trades gases through the shell, one stores its waste, one holds the yolk it feeds on1. Animals that lay it, and their descendants that no longer do, are amniotes.

Soon after that egg appears, in the Carboniferous, the amniotes split: reptiles and birds on one side, the synapsids on the other, which lead to mammals and hold the land for a hundred million years before the first dinosaur.

Warm blood leaves no organ to fossilise. Palaeontologists date it from five indirect clues, read in bone or its chemistry: the density of blood vessels, the speed of growth, the width of the channels feeding the limbs, the temperature the isotopes give, and the shift from a sprawling stance to legs carried under the body, which needs the pressure a four-chambered heart delivers23.

Milk may have begun as water: the parchment shell of the first synapsid eggs appears to have been kept damp by glands on the mother's skin4.

08

Flight, Four Times Over

Flapping flight, kept up by the wings rather than paid for in height, arose four times among animals: once in insects, then in pterosaurs, birds and bats1. Each vertebrate solution builds a wing from different material: the pterosaur stretches a skin membrane along a single enormous finger, the bat between all of its own, and birds carry a curved surface made of feathers.

Feathers long predate flight. They cover dinosaurs with no aerial ability at all, so something else selected for them first, insulation and display being the usual candidates2. Yang and colleagues3 have described filaments on pterosaurs that branch in the way feathers do: either the origin of feathers goes back to the common ancestor of birds and pterosaurs, or those filaments are a second, separate invention.

Among feathered dinosaurs, abandoned solutions sit beside the ones that lasted. One small Jurassic group, the scansoriopterygids, carried skin membranes on greatly elongated forelimbs, a design that disappeared while feathered wings went on1.

09

Alliances Between Kingdoms

Some transitions bind organisms of different kingdoms, plants, animals and fungi, that go on reproducing separately. Flowering plants and their pollinators are the most visible such arrangement on land; underground, more than four fifths of land plants still pair with a fungus1.

Bees are the principal agents of the first. Comparing whole genomes places their origin in the Early Cretaceous, around 128 million years ago2, in western Gondwana3, the single landmass that Africa and South America then formed. Their closest living relatives among the wasps hunt thrips, small insects that gather on flowers and eat pollen, which suggests how hunting turned into harvesting: a wasp carrying prey dusted with pollen was already moving pollen from flower to flower2.

Among those same wasps, and among no others, colonies appeared in which most individuals never breed, and raise another's young instead. Biologists call this eusociality2.

10

Reading Kinship

Every date above comes from one of two sources, and the two are not independent.

Rocks give absolute ages, because certain elements they contain decay at a known rate. A fossil gives only a bound: the group it belongs to already existed, and nothing says for how long it had. Sequences give something else, the shape of the family tree and the relative spacing of its branchings, counted from the differences that pile up between two lineages once they separate.

A tree of relative distances becomes a timetable only when at least one branching point is pinned to a dated rock. That step is called calibration, and it is where dating by sequences is weakest. Pinning only near the tips of the tree, rather than at its deep branchings, can make a whole timescale a thousand times too short1. Beavan and colleagues2 show that pinning a tree after the analysis, instead of building the fossil constraints into the analysis itself, almost always returns the wrong rates and the wrong dates.