The origin of life and the evolution of multicellular organisms

1A sealed flask that changed science

For more than two thousand years many people believed that living things could arise directly from non-living matter: maggots from rotting meat, mice from stored grain, microbes from broth. In the 1860s Louis Pasteur boiled broth in flasks whose long necks he had bent into an S shape. Air could reach the broth, but dust and microbes settled in the bend. The broth stayed clear for months. Only when he broke a neck off did the broth cloud with life. Pasteur had shown that life comes from life. But that raised a deeper question: if life always comes from life, where did the first life come from? This lesson compares the main hypotheses and then traces how simple cells became the multicellular organisms of today.

2Hypotheses on the origin of life

3From chemicals to cells

In the 1920s Alexander Oparin in Russia and J. B. S. Haldane in Britain independently proposed that the early Earth's atmosphere contained little or no free oxygen. Energy from lightning, ultraviolet light and volcanic heat could then have driven reactions between simple gases to make organic molecules, which collected in the oceans as a "primordial soup". Without oxygen, and with no living things to eat them, such molecules would not have been broken down.

In 1953 Stanley Miller, working with Harold Urey, tested the idea:

After a week of sparking, up to 15% of the carbon had been turned into organic compounds, including several amino acids, the building blocks of proteins. Later experiments with different gases, including the carbon dioxide and nitrogen that many scientists now think were more abundant in the early atmosphere, also produced amino acids, sugars and the bases of nucleotides, especially around hot volcanic springs.

Several further steps are needed, and each has been partly reproduced in the laboratory:

1. Polymers. Monomers join into chains, for example on the surfaces of clay minerals, which can hold them close together.
2. Self-replication. In the RNA world hypothesis, the first genetic molecule was RNA, because RNA can both store information and act as an enzyme (a ribozyme), so it could copy itself. DNA and proteins took over these jobs later.
3. Membranes. Lipid molecules in water spontaneously form tiny hollow spheres. One that enclosed self-copying RNA would be a protocell, able to keep its chemicals together and to be acted on by natural selection.

4From the first cells to multicellular life

The Earth formed about 4.6 billion years ago. Chemical traces in ancient rocks, and fossil stromatolites, layered mounds built by mats of microbes, show that prokaryotic life existed by about 3.5 billion years ago. The main stages that followed were:

1. Photosynthesis and oxygen. Cyanobacteria began releasing oxygen by photosynthesis. By about 2.4 billion years ago oxygen started to build up in the atmosphere, which allowed aerobic respiration, with its much greater ATP yield, and later formed the ozone layer that shields the land from ultraviolet radiation.
2. Eukaryotic cells, by about 2 billion years ago. According to the endosymbiotic theory, a larger cell engulfed aerobic bacteria that survived inside it and became mitochondria; in the ancestors of algae and plants, engulfed cyanobacteria became chloroplasts. The evidence: both organelles have two membranes, their own circular DNA without histones, ribosomes of the small (70S) bacterial type, and they divide by a process like binary fission.
3. Multicellularity, by about 1.2 billion years ago in algae. Cells that stayed together after dividing, stuck to each other and signalled to each other could specialise: some for feeding, some for reproduction, some for protection. This division of labour allowed larger size and new ways of life.
4. The Cambrian explosion, from about 538 million years ago: within a few tens of millions of years, representatives of most animal phyla appear in the fossil record, many with shells and skeletons that fossilise well.
5. Life on land: plants by about 470 million years ago, followed by arthropods and then vertebrates; the first mammals about 200 million years ago; and modern humans, in Africa, about 300 000 years ago.