1The time frame: what the rocks say

Scientists date Earth and its oldest rocks by radioactive decay (topic 7.6). The key dates the CED gives:

Reading the window. Life cannot have arisen before conditions allowed it (3.9 bya) and must have arisen before its first fossils (3.5 bya).

Two cautions. The fossil date is a latest possible date for the origin, not the origin itself: older life may have left no fossils, or fossils that were destroyed when the rock was heated and deformed. And the dates are approximate; the exam uses the CED's round figures.

Some of the oldest evidence is stromatolites: layered rock mounds built up as mats of microbes trapped sediment, layer on layer. Similar mounds still form today in a few warm, salty, shallow seas, which helps scientists recognise the ancient ones.

Life arose after Earth became habitable (about 3.9 bya) and before the oldest fossils (about 3.5 bya); everything else in this topic is about how.

2From chemistry to the first cells: testing models

The early Earth had little or no free oxygen in its atmosphere and plenty of energy sources: lightning, ultraviolet light, volcanic heat, and hot water from vents on the sea floor. Models of the origin of life propose that, under such conditions, simple inorganic molecules reacted to form organic molecules such as amino acids and nucleotides, which then joined into polymers, which then came to be enclosed in membranes.

These models are science because they make predictions that can be tested. The best-known test was carried out by Stanley Miller and Harold Urey in 1953. They sealed water, methane, ammonia and hydrogen in glass apparatus, heated the water, and passed electric sparks through the gases to imitate lightning. Within days, the liquid contained amino acids.

Practise the skill on that design:

• Null hypothesis: passing sparks through the gas mixture has no effect on the formation of amino acids.
• Control: an identical apparatus with the same gases and heating but no sparks.
• Prediction if the model is right: amino acids form in the sparked apparatus but not, or far less, in the control.

Later experiments with other gas mixtures, and with conditions like those at sea-floor vents, have also produced organic molecules, and lipids in water form small hollow spheres on their own, which suggests how the first membrane-bound compartments might have formed.

3The chicken-and-egg problem

In every cell today, there is a loop:

• DNA stores the instructions for making proteins;
• proteins (enzymes) are needed to copy DNA and to make RNA and proteins.

So which came first? Information needs catalysts to be copied, and catalysts need information to be made. A first living system that needed both DNA and proteins from the start would need two complicated things to arise at the same moment.

4The RNA world hypothesis

The RNA world hypothesis proposes that the first genetic material was RNA, and that RNA itself did the catalysis, so one kind of molecule broke the loop.

RNA can do both jobs:

• It stores information. The sequence of bases along an RNA strand is information, just as in DNA.
• It can be copied by base pairing. A strand can act as a template: each base pairs with its partner (A with U, G with C), so a complementary strand forms, and copying the copy gives back the original.
• It can catalyse reactions. A single strand of RNA folds into complex three-dimensional shapes, like a protein. Some folded RNAs, called ribozymes, speed up chemical reactions.

The hypothesis rests on three assumptions, which the CED lists. In our words:

1. Genetic continuity came from copying RNA. At some point, information was passed from one generation of molecules to the next by replicating RNA, with no DNA involved.
2. Replication needed base pairing. Copying worked by complementary bases lining up on a template, as in all nucleic acid copying today.
3. No genetically encoded proteins were the catalysts. The reactions, including RNA copying, were catalysed by RNA (or happened without enzymes), not by proteins made from genes.

5The evidence that fits

• Ribozymes exist in cells today. The part of the ribosome that joins amino acids into a polypeptide is made of rRNA, not protein. Every protein in every cell is assembled by an RNA catalyst. Some introns can cut themselves out of RNA without any protein.
• RNA sits in the middle of everything. Cells use RNA to carry information (mRNA), to deliver amino acids (tRNA) and to build the ribosome (rRNA). Many key small molecules, such as ATP, are built from ribonucleotides. That central position looks like a legacy of a time when RNA did everything.
• RNA catalysts can be evolved in the laboratory. Scientists have produced RNA molecules that join nucleotides together to copy short RNA templates, showing that RNA can, in principle, help replicate RNA.

Why did DNA and proteins take over? DNA is double-stranded and lacks the reactive –OH group on its sugar that RNA has, so it is more chemically stable, a better long-term store. Proteins, built from twenty kinds of amino acid rather than four kinds of nucleotide, can fold into a wider range of shapes and are usually better catalysts. The hypothesis proposes that RNA came first and was then partly replaced.