How Did Life Begin on Earth? What Science Can and Cannot Say
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Nobody knows how life began, and it is worth saying so before anything else. What science has is a set of well-supported pieces: when it happened, what the early Earth was like, which chemical steps have been shown to work in the laboratory, and where the gaps remain. The pieces do not yet join into a single story, but they narrow the possibilities more than most people expect.
When: earlier than seems reasonable
The Earth formed about 4.54 billion years ago and was uninhabitable for its first stretch: molten, bombarded, and struck by a Mars-sized body that made the Moon. Liquid water oceans existed by around 4.4 billion years ago, judging by the chemistry of the oldest surviving crystals. The oldest widely accepted fossils are microbial mats called stromatolites from Australia, about 3.5 billion years old, and there are chemical traces in rocks from Greenland and Canada that some researchers argue push life back to 3.8 billion years or more.
That leaves a window of a few hundred million years between the first oceans and the first clear life, and possibly much less. Whatever happened, it did not take long on a geological scale, which suggests that it was not an outrageous fluke.
The building blocks are easy
In 1953 Stanley Miller sealed water, methane, ammonia and hydrogen in a flask, sparked it with electricity to imitate lightning, and found amino acids, the components of proteins, in the water a week later. His atmosphere was probably wrong, the early Earth's was richer in carbon dioxide and nitrogen, but repeats with more realistic mixtures still produce amino acids, and so do meteorites: the Murchison meteorite that fell on Australia in 1969 contained dozens of them, along with sugars and the bases used in DNA and RNA. The components of life form readily wherever there is carbon, energy and water.
The difficulty is not making the parts. It is getting them to assemble into something that copies itself, and that is where the field divides.
RNA first
Living cells face a chicken-and-egg problem: DNA stores the instructions but needs proteins to copy it, and proteins are made from DNA's instructions. The most widely held answer is that neither came first. RNA, DNA's single-stranded cousin, can both carry information and, as Thomas Cech and Sidney Altman showed in the 1980s, act as a catalyst that speeds up chemical reactions. A world of self-copying RNA molecules could have preceded both DNA and protein.
Support has grown steadily. Laboratory evolution has produced RNA molecules that copy other RNA, though not yet themselves completely. John Sutherland's group in Cambridge showed in 2009 a plausible route from simple chemicals, hydrogen cyanide and its relatives, to RNA's building blocks under conditions resembling a drying pond struck by ultraviolet light. And the ribosome, the machine in every living cell that builds proteins, turns out to be made mostly of RNA doing the actual work, a fossil of the RNA world still running inside us.
Where: vents, ponds or something else
Two settings compete. Hydrothermal vents on the ocean floor, particularly the alkaline vents discovered in 2000 at a site called the Lost City, produce warm, mineral-rich fluid flowing through porous rock towers, with natural chemical gradients across thin mineral walls that resemble the gradients living cells use to make energy. Nick Lane and others argue that such a vent supplies the energy, the confinement and the catalysts that a drying pond does not.
The pond camp replies that the key chemistry, especially making RNA and joining its units into chains, needs cycles of wetting and drying, concentrated solutions and ultraviolet light, none of which exist at the bottom of the sea. Volcanic hot springs on land, like those in Yellowstone or Iceland today, give all three. Charles Darwin guessed at a warm little pond in a letter of 1871. It is still a live option, and the honest position is that both settings have chemistry in their favour and neither has been made to work end to end.
What every cell remembers
Whatever the route, it converged. All life on Earth shares the same genetic code, the same set of twenty amino acids in proteins, the same handedness of its molecules and the same core metabolism, which means everything alive descends from one population of early cells, called the last universal common ancestor. Comparing the genes shared across all three domains of life suggests that ancestor already lived in a hot, mineral-rich, oxygen-free environment and used hydrogen and carbon dioxide for energy, which fits the vent picture, though the argument is far from settled. What is established, what is supported and what is open:
- •Established: life existed by 3.5 billion years ago, within a few hundred million years of the oceans
- •Established: amino acids, sugars and nucleobases form easily on planets and in space
- •Well supported: an RNA-based stage came before DNA and proteins
- •Contested: whether it happened at deep-sea vents or in ponds on land
- •Open: how the first self-copying system arose and became a cell
The takeaway
Life on Earth appeared quickly once there was water, its building blocks form easily, and an RNA world very probably preceded the DNA-and-protein system every cell now uses. Whether it started at a deep-sea vent or in a pond on land, and how the first molecules began to copy themselves, remain unsolved, which is the difference between a mystery and a research programme.