AP BiologyUnit 7: Natural Selection7.12
AP Biology · Unit 7 · Topic 7.12

Origins of Life on Earth

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson4 min read

Why RNA first? It can carry information and catalyze reactions, the two jobs DNA and proteins split today.

Life probably began as chemistry, not as a finished cell. Organic molecules formed without life, linked into chains, and got wrapped in simple membranes. RNA, which can both store information and speed up reactions, was likely the first genetic material.

Key terms

  • Abiotic synthesis — making organic molecules by chemistry alone, with no living thing involved.
  • Miller–Urey experiment — in 1953, sparks through water vapor, methane, ammonia, and hydrogen made amino acids within a week.
  • Protocell — a membrane bubble, often of fatty acids, that keeps molecules inside. A possible step toward cells.
  • Ribozyme — an RNA molecule that acts as a catalyst. The part of the ribosome that joins amino acids is one.
  • RNA world — the hypothesis that RNA first did both jobs: storing information and catalyzing reactions. DNA and proteins took over later.

How it works

  1. Earth formed about 4.6 billion years ago. Its early air held almost no free oxygen, and lightning, UV light, and volcanic heat supplied energy.
  2. Simple organic molecules formed: amino acids, sugars, nitrogen bases. With no oxygen to break them down, they built up. Meteorites delivered more.
  3. Monomers joined into polymers on mineral surfaces such as clay. The surface gathers monomers and holds them close, so chains grow longer than in open water.
  4. Fatty acids self-assembled into hollow protocells. These grow by taking in more fatty acid, and big ones split under gentle force, with no enzymes.
  5. In the RNA world model, RNA inside stored information in its sequence and, as ribozymes, catalyzed reactions. A dividing protocell passed RNA to both halves.
  6. Now selection could act: RNA sequences that copied faster became common. Later, DNA took over storage because it is more stable, and proteins took over most catalysis.

Why it matters

Origins questions are evidence questions. Given a flask, clay, vesicles, or a meteorite, say what the data show and no more. The oldest widely accepted fossil microbes are about 3.5 billion years old, so this chemistry came earlier. It ties back to Unit 6: the ribosome's catalytic core is RNA, widely seen as a leftover of the RNA world.

Exam traps

TrapActually
RNA came first because it is more stable than DNA.RNA is less stable than DNA. Its case rests on doing two jobs at once: carrying information and catalyzing reactions.
The first cells needed oxygen, so life waited for it to build up.Free oxygen would have broken down new organic molecules. It built up later, from photosynthesis, after life had begun.
Amino acids inside a meteorite mean it carried life.Nonliving chemistry makes L and D mirror forms equally; life's proteins use almost only L. An even mix points to chemistry, not organisms.
Clay supplies the atoms or the sequence of the first chains.Clay only gathers monomers and holds them in place. It adds no atoms and no sequence.
Worked example

Researchers make a pool of about a hundred trillion random RNA sequences. Each round, they keep only the molecules that managed to join a short RNA piece onto their own end. They copy those keepers using protein enzymes, which make occasional errors, and repeat. After 10 rounds, the pool performs the joining reaction about 1,000 times faster than the starting pool. Which conclusion about early life do these results best support?

  1. Name what was selected: the joining reaction. That is catalysis, and RNA did it with no protein in the reaction.
  2. Name what was inherited: the base sequence. Keepers were copied, so their sequences, plus a few errors, made up the next round.
  3. Put them together. Variation, selection, and inheritance improved the catalyst round after round, and one molecule held both the information and the chemistry.
  4. Check what the researchers supplied. Protein enzymes did the copying, so these data do not show RNA copying itself.

RNA can carry heritable information and act as a catalyst at the same time, and selection can improve its catalysis. That is the core of the RNA world hypothesis.

The sequence was both the thing copied and the thing that did the chemistry, so one molecule did the two jobs that DNA and proteins split today. The near-miss is 'RNA can replicate itself without proteins.' Protein enzymes did the copying here, so that claim goes beyond the data.

Now try it on real exam-style questions

Ten questions on Origins of Life on Earth, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 7.12 Origins of Life on Earth

Many biologists propose that the earliest genetic material on Earth was made of which molecules?

  1. A Short DNA strands
  2. B Short protein chains
  3. C Short RNA strands
  4. D Short lipid chains
Show the answer

C — The RNA world hypothesis holds that RNA came first because RNA can both carry information and catalyze reactions. DNA is the genetic material of modern cells, but it is thought to have taken over that role later.

Question 2 · 7.12 Origins of Life on Earth

Compared with today's atmosphere, early Earth's atmosphere held very little of which gas?

  1. A Carbon dioxide (CO2)
  2. B Nitrogen (N2)
  3. C Water vapor (H2O)
  4. D Oxygen (O2)
Show the answer

D — Free O2 was scarce on early Earth; it built up only after photosynthetic prokaryotes had been releasing it for a long time. Carbon dioxide was actually more abundant then than now, so it is the wrong choice despite being a gas that changed a great deal.

Question 3 · 7.12 Origins of Life on Earth

About how old is Earth, based on radiometric dating of the oldest rocks and meteorites?

  1. A About 3.5 billion years
  2. B About 4.6 million years
  3. C About 4.6 billion years
  4. D About 540 million years
Show the answer

C — Radiometric dating puts Earth's formation at about 4.6 billion years ago. The 3.5-billion-year figure is the age of the oldest widely accepted fossils, which formed about a billion years after the planet itself.