1. CHAPTER 26 Early Earth and the Origin of Life Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Section A: Introduction to the History of Life 1.Life on Earth originated between 3.5 and 4.0 billion years ago 2.Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago 3. Oxygen began accumulating in the atmosphere about 2.7 billion years ago 4. Eukaryote life began by 2.1 billion years ago 5. Multicellular eukarotes evolved by 1.2 billion years ago 6. Animal diversity exploded during the early Cambrian period 7. Plants, fungi, and animals colonized the land about 500 million years ago

2. Life is a continuum extending from the earliest organisms through various phylogenetic branches to the great variety of forms alive today. The diversification of life on Earth began over 3.8 billion ago. Introduction Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

3. Geologic events that alter environments have changed the course of biological evolution. For example, the formation and subsequent breakup of the supercontinent Pangea has a tremendous impact on the diversity of life. Conversely, life has changed the planet it inhabits. The evolution of photosynthetic organisms that release oxygen into the air had a dramatic impact on Earth’s atmosphere. Much more recently, the emergence of Homo sapiens has changed the land, water, and air on a scale and on a rate unprecedented for a single species. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

4. Historical study of any sort is an inexact discipline that depends on the preservation, reliability, and interpretation of past records. The fossil record of past life is generally less and less complete the farther into the past we delve. Fortunately, each organism alive today carries traces of its evolutionary history in its molecules, metabolism, and anatomy. Still, the evolutionary episodes of greatest antiquity are the generally most obscure. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

5. One can view the chronology of the major episodes that shaped life as a phylogenetic tree. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.1

6. Alternatively, we can view these episodes with a clock analogy. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.2

7. For the first three-quarters of evolutionary history, Earth’s only organisms were microscopic and mostly unicellular. The Earth formed about 4.5 billion years ago, but rock bodies left over from the origin of the solar system bombarded the surface for the first few hundred million years, making it unlikely that life could survive. No clear fossils have been found in the oldest surviving Earth rocks, from 3.8 billion years ago. 1. Life on Earth originated between 3.5 and 4.0 billion years ago Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

8. The oldest fossils that have been uncovered were embedded in rocks from western Australia that are 3.5 billion years ago. The presence of these fossils, resembling bacteria, would imply that life originated much earlier. This may have been as early as 3.9 billion years ago, when Earth began to cool to a temper- ature at which liquid water could exist. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.3a

9. Prokaryotes dominated evolutionary history from about 3.5 to 2.0 billion years ago. The fossil record supports the hypothesis that the earliest organisms were prokaryotes. Relatively early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea. Representatives from both groups thrive in various environments today. 2. Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

10. Two rich sources for early prokaryote fossils are stromatolites (fossilized layered microbial mats) and sediments from ancient hydrothermal vent habitats. This indicates that the metabolism of prokaryotes was already diverse over 3 billion years ago. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.4

11. Photosynthesis probably evolved very early in prokaryotic history. The metabolism of early versions of photosynthesis did not split water and liberate oxygen. 3. Oxygen began accumulating in the atmosphere about 2.7 billion years ago Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

12. Cyanobacteria, photosynthetic organisms that split water and produce O 2 as a byproduct, evolved over 2.7 billion years ago. This early oxygen initially reacted with dissolved iron to form the precipitate iron oxide. This can be seen today in banded iron formations. About 2.7 billion years ago oxygen began accumulating in the atmosphere and terrestrial rocks with iron began oxidizing. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

13. While oxygen accumulation was gradual between 2.7 and 2.2 billion years ago, it shot up to 10% of current values shortly afterward. This “corrosive” O 2 had an enormous impact on life, dooming many prokaryote groups. Some species survived in habitats that remained anaerobic. Other species evolved mechanisms to use O 2 in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

14. Eukaryotic cells are generally larger and more complex than prokaryotic cells. In part, this is due to the apparent presence of the descendents of “endosymbiotic prokaryotes” that evolved into mitochondria and chloroplasts. 4. Eukaryotic life began by 2.1 billion years ago Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

15. While there is some evidence of earlier eukaryotic fossils, the first clear eukaryote appeared about 2.1 billion years ago. Other evidence places the origin of eukaryotes to as early as 2.7 billion years ago. This places the earliest eukaryotes at the same time as the oxygen revolution that changed the Earth’s environment so dramatically. The evolution of chloroplasts may be part of the explanation for this temporal correlation. Another eukaryotic organelle, the mitochondrion, turned the accumulating O 2 to metabolic advantage through cellular respiration. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

16. A great range of eukaryotic unicellular forms evolved into the diversity of present-day “protists.” Multicellular organisms, differentiating from a single-celled precursor, appear 1.2 billion years ago as fossils, or perhaps as early as 1.5 billion years ago from molecular clock estimates. 5. Multicellular eukaryotes evolved by 1.2 billion years ago Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.6

17. Recent fossil finds from China have produced a diversity of algae and animals from 570 million years ago, including beautifully preserved embryos. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.7

18. Geologic evidence for a severe ice age (“snowball Earth” hypothesis) from 750 to 570 million years ago may be responsible for the limited diversity and distribution of multicellular eukaryotes until the very late Precambrian. During this period, most life would have been confined to deep-sea vents and hot springs or those few locations where enough ice melted for sunlight to penetrate the surface waters of the sea. The first major diversification of multicellular eukaryotic organisms corresponds to the time of the thawing of snowball Earth. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

19. A second radiation of eukaryotic forms produced most of the major groups of animals during the early Cambrian period. Cnidarians (the phylum that includes jellies) and poriferans (sponges) were already present in the late Precambrian. 6. Animal diversity exploded during the early Cambrian period Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

20. However, most of the major groups (phyla) of animals make their first fossil appearances during the relatively short span of the Cambrian period’s first 20 million years. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 26.8

21. The colonization of land was one of the pivotal milestones in the history of life. There is fossil evidence that cyanobacteria and other photosynthetic prokaryotes coated damp terrestrial surfaces well over a billion years ago. However, macroscopic life in the form of plants, fungi, and animals did not colonize land until about 500 million years ago, during the early Paleozoic era. 7. Plants, fungi, and animals colonized the land about 500 million years ago Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

22. The gradual evolution from aquatic to terrestrial habitats required adaptations to prevent dehydration and to reproduce on land. For example, plants evolved a waterproof coating of wax on the leaves to slow the loss of water. Plants colonized land in association with fungi. Fungi aid the absorption of water and nutrients from the soil. The fungi obtain organic nutrients from the plant. This ancient symbiotic association is evident in some of the oldest fossilized roots. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

23. Plants created new opportunities for all life, including herbivorous (plant-eating) animals and their predators. The most widespread and diverse terrestrial animals are certain arthropods (including insects and spiders) and certain vertebrates (including amphibians, reptiles, birds, and mammals). Most orders of modern mammals, including primates, appeared 50-60 million years ago. Humans diverged from other primates only 5 million years ago Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

24. The terrestrial vertebrates, called tetrapods because of their four walking limbs, evolved from fishes, based on an extensive fossil record. Reptiles evolved from amphibians; both birds and mammals evolved from reptiles. Most orders of modern mammals, including primates, appeared 50-60 million years ago. Humans diverged from other primates only 5 million years ago. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings