History of life and human evolution
Evolution of plants and vertebrates through geological time; origin and evolution of humans.
The essentials
About 2000 million years ago (mya), the first cellular forms of life appeared on Earth. Some of these cells could release oxygen, probably similar to photosynthesis. Slowly, multicellular organisms evolved. By about 500 mya, invertebrates had formed and were active. Jawless fish probably evolved around 350 mya. Seaweeds and a few plants existed probably around 320 mya. The first organisms to invade land were plants, and they were widespread on land when animals invaded it.
Fish with stout and strong fins could move onto land and go back to water; these were lobefins, which evolved into the first amphibians. A lobefin fish, the coelacanth, caught in South Africa in 1938, was thought to be extinct. Amphibians evolved into reptiles, which lay thick-shelled eggs that do not dry up in the sun. Some reptiles returned to water and evolved into fish-like reptiles (Ichthyosaurs). The land reptiles included dinosaurs; the biggest, Tyrannosaurus rex, was about 20 feet tall. About 65 mya, dinosaurs suddenly disappeared. Mammals, which were viviparous and protected their unborn young inside the mother's body, spread widely. In plants, seed ferns and gymnosperms preceded the flowering plants.
Human evolution: about 15 mya, primates called Dryopithecus and Ramapithecus existed; they were hairy and walked like gorillas and chimpanzees. Ramapithecus was more man-like, while Dryopithecus was more ape-like. Fossils from Ethiopia and Tanzania show hominid features. About 2 mya, Australopithecines probably lived in East African grasslands; they hunted with stone weapons and essentially ate fruit. Homo habilis, probably the first human-like being, had a brain capacity of 650–800 cc and probably did not eat meat. Fossils discovered in Java in 1891 revealed Homo erectus about 1.5 mya, with a large brain of around 900 cc, which probably ate meat. The Neanderthal man, with a brain size of about 1400 cc, lived in near-east and central Asian countries between 1,00,000 and 40,000 years back; they used hides to protect their bodies and buried their dead. Homo sapiens arose in Africa and moved across continents. Pre-historic cave art developed about 18,000 years ago. Agriculture came around 10,000 years back, and human settlements started.
Exam tips
Memorise the brain capacities: Homo habilis 650–800 cc, Homo erectus about 900 cc, Neanderthal about 1400 cc.