The first fossil evidence of cells with nuclei and internal structure
like eucaryotes comes from dolomite rock from Beck Springs, California.
These rocks are 1.4 billion to 1.2 billion years old (shown on
previous page). From this time on, the evidence is increasingly
solid. The changeover of the atmosphere to oxidizing conditions,
the development of enough O2-respiring
procaryotes to show up plentifully in the fragmentary fossil record,
and the development of eucaryotic cells, all apparently took place
1.8 to 1.3 billion years ago.
As an interesting sidelight to this chronology, one can compare
the amino acid sequences from a protein that is present in many
forms of life to obtain a rough measure of how distantly related
these forms are, and how long ago their ancestors diverged.
The sequences of respiratory cytochrome c from more than 67 eucaryotic
species have been compared, including vertebrates of all kinds,
insects, microorganisms, and higher plants.
Examination of the rates at which the cytochromes change in different
lines of descent suggests that plants and animals diverged approximately
1.2 billion years ago, in excellent agreement with the fossil evidence
for early eucaryotes.