1. CHAPTER 26 EARLY EARTH AND THE ORIGIN OF LIFE
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2. 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.
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3. One can view the chronology of the major episodes that shaped life as a phylogenetic tree.
Fig. 26.1
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4. Alternatively, we can view these episodes with a clock analogy.
Fig. 26.2
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5. 1. Life on Earth originated between 3.5 and 4.0 billion years ago
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.
Origin of life
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6. 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 temperature at which liquid water could exist).
Fig. 26.3a
fossil
living

7. 2. Prokaryotes dominated evolutionary history from 3. 5 to 2
2. Prokaryotes dominated evolutionary history from 3.5 to 2.0 billion years ago
Prokaryotes dominated evolutionary history from about 3.5 to 2.0 billion years ago.
Relatively (very?) early, prokaryotes diverged into two main evolutionary branches, the bacteria and the archaea.
Prokaryotes dominate
3,5-2,0 billion years ago
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8. Early prokaryote fossils - stromatolites (fossilized layered microbial mats)
This indicates that the metabolism of prokaryotes was already diverse over 3 billion years ago.
Fig. 26.4
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9. 3. Oxygen began accumulating in the atmosphere about 2
3. Oxygen began accumulating in the atmosphere about 2.7 billion years ago
Photosynthesis probably evolved very early in prokaryotic history.
The metabolism of early versions of photosynthesis did not split water and liberate oxygen.
2,7 bill years ago

10. Cyanobacteria, photosynthetic organisms that split water and produce O2 as a byproduct, evolved over 2.7 billion years ago.
Cyanobacterial
fossils
Living
cyanobacteria

11. While oxygen accumulation was gradual between 2. 7 and 2
While oxygen accumulation was gradual between 2.7 and 2.2 billion years ago, it shot up to 10% of current values shortly afterward.
Why? Evolution of eukaryotic algae?
This “corrosive” O2 had an enormous impact on life, dooming many prokaryote groups.
Some species survived in habitats that remained anaerobic.
Other species evolved mechanisms to use O2 in cellular respiration, which uses oxygen to help harvest the energy stored in organic molecules.
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12. 4. Eukaryotic life began by 2.1 billion years ago
Eukaryotic cells are generally larger and more complex than prokaryotic cells.
They contain the descendents of “endosymbiotic prokaryotes” that evolved into mitochondria and chloroplasts.
Nucleus
Mitochondrion
Chloroplast
Chromosomes
Eukaryotes
Bacterium
DNA
2.1 bill years ago

13. The origin of eukaryotes (see Chapt 28)
2,1 bill years ago
The origin of eukaryotes (see Chapt 28)
Gene transfer – from
prokaryote to host nucleus
Gene transfer –
from prokaryote to host nucleus

14. The earliest eukaryotes evolved 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 O2 to metabolic advantage through cellular respiration.
Nucleus
Mitochondrion
Chloroplast
Eukaryotic cell
Chromosomes
Eukaryotes: animals, plants, fungi

15. 5. Multicellular eukaryotes evolved by 1.2 billion years ago
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.
Fig. 26.6
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16. Recent fossils finds from China have produced a diversity of algae and animals from 570 million years ago, including beautifully preserved embryos.
Fig. 26.7
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17. 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.
The first major diversification of multicellular eukaryotic organisms corresponds to the time of thawing of snowball Earth.
570 mill years ago
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18. 6. Animal diversity exploded during the early Cambrian period
A second radiation of eukaryotic forms produced most of the major groups of animals during the early Cambrian period.
“The Cambrian explosion”
Cnidarians (the plylum that includes jellies) and poriferans (sponges) were already present in the late Precambrian.
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19. 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.
Fig. 26.8
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20. 7. Plants, fungi, and animals colonized the land about 500 million years ago
The colonization of land was one of the pivotal milestones in the history of life.
500 mill

21. Plants colonized land in association with fungi.
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.
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22. Humans diverged from other primates only 5 million years ago
Plants created new opportunities for all life, including herbivorous (plant-eating) animals and their predators.
Most orders of modern mammals, including primates, appeared million years ago.
Humans diverged from other primates only 5 million years ago
Humans
diverged from
primates 5 mill
years ago
Modern mammals
50-60 mill years ago
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23. 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

24. 1. The five-kingdom system reflected increased knowledge of life’s diversity
Traditionally, systematists have considered kingdom as the highest taxonomic category.
R.H Whittaker, 1969 –
five kingdom system

25. 2. Arranging the diversity of life into the highest taxa is a work in progress
During the last three decades, systematists applying cladistic analysis, including the construction of cladograms based on molecular data, have been identifying problems with the five- kingdom system.
These data led to the three-domain system: 1) Bacteria, 2) Archaea, and 3)Eukarya, as superkingdoms.
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26. Many microbiologists have divided the two prokaryotic domains into multiple kingdoms based on cladistic analysis of molecular data.
Present view
Fig
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27. A second challenge to the five kingdom system comes from systematists who are sorting out the phylogeny of the former members of the kingdom Protista.
Molecular systematics and cladistics have shown that the Protista is not monophyletic.
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28. The multi-kingdom scenario fits the
phylogenetic tree