Life evolved in the oceans about 3.8 billion years ago. It diversified as atmospheric oxygen approached its current level. Numerous climate changes and rearrangements of the continents, as well as meteorite impacts, contributed to five major mass extinctions.
learning outcomes
You should be able to:
Explain how increases and decreases in biodiversity through time relate to major changes in Earth’s environmental conditions.
Characterize environmental conditions during successive geological periods and connect these periods with major events in biological evolution.
Illustrate the relationship between a phylogenetic tree and geological time.
Give a likely reason for the “green revolution” of the Cenozoic era, and explain why it enabled flowering plants to diversify.
A major factor was the evolution of two genes in legumes that allowed these plants to use atmospheric nitrogen directly by forming symbioses with a few species of nitrogen-
Compare the tree of life in Appendix A with the chronology of major events in Table 24.1. To represent the diversity of life in one figure, the tree in the appendix is not drawn proportionally in relation to time. Which geological eon, era, or period seems most distorted, and why? What major changes to the tree would be needed to show divergences in groups that are proportional?
The Archean and Proterozoic eons lasted about 1.3 billion and 2.0 billion years, respectively, compared with the 0.542 billion years of the Phanerozoic eon. Yet most of the tree depicts events in the much shorter Phanerozoic eon, since most of the major groups of life shown on the tree diverged during this time. If the tree depicted time proportionally, the branches at the base of the tree would need to be much longer. However, if this change were made, the divergence of the eukaryotes would be so compressed that their relationships could not be shown clearly on one page.