1. The solar system formed about 4.55 billion years ago, and life left evidence on Earth by about 700 million years after that. Conditions when life originated differed from today's environment.
3. Prebiotic simulations combine simple chemicals that include the elements in organisms in the presence of energy. Organic building blocks form, including simple carbohydrates, amino acids, and nucleotides.
4. Amino acids may have polymerized into peptides and nucleotides into nucleic acids on hot clay or mineral surfaces that served as templates, structural supports, and supplied energy and catalysis.
5. The RNA world theory proposes that RNA or similar but more stable molecule proceded formation of the first cells because it could repicate, encode information, change, and catalyze reactions. Proteins, with their tremendous variety, provided more enzymes and structural features. Reverse transcriptase could have copied RNA's information into more permanent DNA.
6. Phospholipid sheets that formed bubbles around proteins and nucleic acids may have formed cell precursors, or progenotes.
7. Metabolic pathways may have originated when progenotes mutated in ways that enabled them to use alternate or additional nutrients.
18.2 Highlights in the History of Life
8. Life began in the seas. The earliest fossils are of cyanobacteria from 3.7 billion years ago. The oldest eukaryotic fossils, of algae, date from 1.9 billion years ago. Evidence of multicellularity dates to 1.2 billion years ago.
9. The Ediacarans were soft, flat organisms completely unlike modern species. They lived in the late Precambrian and early Cambrian periods.
10. The Cambrian explosion introduced evidence of many species, notably those with hard parts. Amphibian-like animals ventured onto land about 360 million years ago. By 300 million years ago, reptiles had appeared and then diverged, eventually evolving also into birds and mammals. Invertebrates, ferns, and forests flourished. The Paleozoic era ended with mass extinctions.
11. Dinosours prevailed throughout the Mesozoic era, when forests were largely cycads, ginkgos, and conifers. In the middle of the era, flowering plants became prevalent. When the dinosaurs died out 65 million years ago, resources opened up for mammals.
12. Molecular evidence dates the origin of mammals to 100 million years ago, and fossil evidence indicates their adaptive radiation beginning 65 million years ago. Placental mammals eventually replaced many marsupial species.
13. Mammalian species changed throughout the Cenzoic as the climate cooled and dried.
14.
Aegyptopithecus
and other primates preceded the hominoids, which were ancestral to apes and humans. Hominids were ancestral to humans only. Four million years ago several species of
Australopithecus
existed and were gradually replaced with
Homo habilis
and then
Homo erectus. Cro-Magnons were
Homo sapiens, as are modern humans. The Neanderthals lived at the same time as early
Homo sapiens, but were probably a separate species.
