The Origin and History of Life
|18.1 How Might Life Have Originated
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 orignated differed from today.
2. When life began, Earth was geologically unstable, and the atmosphere was high in hydrogen and low oxygen.
3. Prebiotic simulations combine simple chemicals that include teh elements in organisms in the presence of energy. Organic building blocks form, some of which are also in organisms.
4. Amino acids may have polymerized into peptides and nucleotides into nucleic acids on hot clay or mineral surfaces that servedas 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 became enzymes. Reverse transcriptase could have copied RNA's information into more permanent DNA.
6. Phospholipid sheets that formed bubbles around protiens 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 addtional 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 eukarytic 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 unlike modern species that 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 appeard and then diverged, eventually evolving also into birds and mammals. Invertebrates, ferns, and forests flourished. The Paleozoic era ended with mass extinctions.
18.3 The Age of Mammals
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.|