Every tree of life has a root. Endosymbiosis explains where eukaryotic cells came from, and the fossil record shows life changing for billions of years. How the very first cells arose is a different question, and one that science is still working on. Here is what is well established, and what is still open.
Key idea
Earth formed about 4.5 billion years ago, and fossils of microbial life go back about 3.5 billion years. Experiments show that simple organic molecules, such as amino acids, form under early-Earth conditions. How the first cells arose is still an open question, studied through hypotheses such as an "RNA world".
- Building blocks form readily. In 1952 Stanley Miller, working with Harold Urey, passed electric sparks through methane, ammonia, hydrogen and water vapor and found amino acids. Geologists now think the early atmosphere had less of those gases, but similar experiments with other mixtures, and amino acids found in meteorites, show that such molecules can form without life.
- RNA can both store information and catalyze reactions. Ribozymes, including the RNA at the heart of the ribosome, are enzymes made of RNA. That makes it plausible that early life used RNA before DNA and proteins: the RNA world hypothesis.
- Membranes can form on their own. Lipid-like molecules in water assemble into small bubbles (vesicles) with an inside and an outside, a possible start for cells.
Why it works
Why does RNA get so much attention? A living system needs information that can be copied and chemistry that copies it. In cells today, DNA holds the information and proteins do the chemistry, and each needs the other: a chicken-and-egg problem. RNA can do a little of both, so a world where RNA did both jobs could have come first. That is a hypothesis with supporting evidence, not a settled account; scientists also study hot springs and deep-sea vents as possible settings.
Worked examples
Worked example: How long did it take?
About how long after Earth formed do the oldest widely accepted fossils of life appear?
- Earth: about 4.5 billion years ago. Fossils: about 3.5 billion years ago.
- 4.5 − 3.5 = 1.0.
Answer: About 1 billion years, and part of that time Earth was too hot and too heavily bombarded for life to last.
Worked example: What did Miller show?
Which conclusion does the Miller-Urey experiment support?
- It produced amino acids from simple gases and energy.
- It did not produce cells, nor anything that copies itself.
Answer: Some of life's building blocks can form from nonliving chemistry under early-Earth-like conditions.
Common mistake
Thinking the Miller-Urey experiment made living cells, or that the origin of life is settled. It made amino acids. The steps from molecules to the first self-copying cells are an active field of research with several competing hypotheses. The later history, from microbes 3.5 billion years ago to the life around us, is supported by a deep fossil and genetic record.