Chapter 18 THE ORIGIN AND HISTORY OF LIFE

A. The Origin of Life Clues from geology & paleontology provide us with information of Early Earth.

Prebiotic simulations demonstrate how life might have arisen.

A Theory of Life’s Beginnings

How might metabolic pathways have arisen?

B. Major Events of Life’s History

Precambrian Era ] Origin of life ( bya) Earliest prokaryotic fossils are ~ 3.5 billion years old. ] Origin of photosynthesis ( bya) Fossils from Rhodesia contain breakdown products of chlorophyll. ] Origin of eukaryotic cells (1.9 bya) Earliest eukaryote fossils (unicellular algae) are ~ 1.7 billion years old.

Endosymbiont theory - eukaryotic cells arose from ancient cells (perhaps archaea) that engulfed aerobic & photosynthetic bacteria.

] Origin of multicellular organisms (1.2 bya) Fossils of multicellular algae found in Canada & Siberia. Paleozoic Era ] First vertebrates (jawless fishes) appeared during Ordovician period ( mya). ] First land plants & animals appeared during Silurian period ( mya). ] First amphibians appeared in early Carboniferous period ( mya).

] First reptiles appeared in late Carboniferous period (300 mya). Mesozoic Era ] First dinosaurs appeared during Triasic period ( mya). ] First flowering plants & birds appeared during Jurassic period ( mya). ] First mammals appeared during Cretaceous period ( mya).

C. The Age of Mammals Extinction of the dinosaurs 65 mya opened many niches that mammals filled via extensive adaptive radiation. All mammals possess hair & have mammary glands that produce milk. Egg-laying mammals (monotremes) were probably first to evolve, followed by pouched mammals (marsupials) & finally placental mammals.

Placental Mammal Migration

The Road to Humanity ] Primates first appeared 60 mya ] Aegyptopithecus, a monkey like animal, appeared mya; believed to be ancestors of gibbons, apes & humans. ] First hominoids (primates ancestral to apes & humans) believed to have appeared mya. ] First hominids (primates ancestral to humans) believed to have appeared 19-4 mya.

] Australopithecus appeared 4 mya; exhibited bipedalism (walked upright); several species have been identified in Africa; coexisted with Homo habilis. ] Homo habilis (“handy man”) appeared 2 mya; lived in caves, cared for young & exhibited tool use; coexisted with Homo erectus. ] Homo erectus (“upright man”) appeared 1.6 mya; left fossil evidence of social organization, cooperation & use of fire.

H. erectus coexisted with Homo sapiens. ] Homo sapiens appeared 200,000 years ago; had well developed frontal brain regions. ] Neanderthals appeared in Europe 150,000 years ago & coexisted with H. sapiens from 40,000-30,000 years ago; unlikely that Neanderthals & H. sapiens interbred; probably represent an evolutionary dead end.