Evolution Through Time and the Descent of Humans: Quick Revision

A brief note on Evolution Through Time and the Descent of Humans. Read it before a test. Try each question at the end before you open its answer.

The history of life, in order (NCERT dates)

• About 2000 million years ago: first cellular forms. About 500 million years ago: invertebrates active. About 450 million years ago: plants on land.
• About 350 million years ago: fish with strong fins move on land. Lobefins (coelacanth, caught in 1938) gave amphibians.
• Amphibians gave reptiles, which lay thick-shelled eggs. Ichthyosaurs went back to the water (about 200 million years ago).
• About 65 million years ago: the dinosaurs disappeared. The true reason is not known, and birds are present day dinosaurs.
• Mammals: the first were small and shrew-like, and viviparous. They took over when the reptiles declined. Pouched mammals survived in Australia.
• This account by dates is not in the NEET syllabus, but NCERT, A-Level and IB teach it.

The human line (NCERT)

• Dryopithecus (more ape-like) and Ramapithecus (more man-like): about 15 million years ago, hairy.
• Australopithecines: about 2 million years ago, East Africa, stone weapons, mainly fruit.
• Homo habilis: first human-like being, 650 to 800 cc, probably no meat.
• Homo erectus: about 1.5 million years ago, about 900 cc, probably ate meat.
• Neanderthal man: about 1400 cc, 100000 to 40000 years ago, hides, buried the dead.
• Modern Homo sapiens: arose in Africa, 75000 to 10000 years ago. Cave art about 18000 years ago. Agriculture about 10000 years ago.

Trees and clocks (A-Level and IB)

• A tree shows common ancestors. A more recent common ancestor means closer relatives.
• UPGMA: join the closest pair at half the distance, average the distances, repeat.
• Molecular clock: $t = d \div (2r)$, because both lines change.

A worked example

A skull has 1260 cc and is compared with 900 cc. By what percentage is it larger?

1. The increase is $1260 - 900 = 360$ cc.
2. As a share of 900 cc: $360 \div 900 \times 100 = 40$ per cent.

Practice with new numbers

A program to try

The program prints the percentage rise in the brain capacity from one step to the next.

Mistakes to avoid

• Saying that humans came from chimpanzees. They share a common ancestor.
• **Putting Neanderthal man before Homo erectus.** The order is habilis, erectus, Neanderthal.
• Dividing by the later value in a percentage rise. Divide by the earlier value.
• Forgetting the factor 2 in the molecular clock.

Check yourself

1. Which ancestor probably ate meat?

2. Which group gave rise to the amphibians?

3. Who buried the dead and used hides?

4. Two species differ by 8 per cent. Each line changes by 1 per cent in each million years. When did they split?

Sources

• NCERT (2023). Biology, Textbook for Class XII, chapter 6.
• Reece J. B. et al. (2017). Campbell Biology, 11th edition, Pearson, chapters 25 and 34.