ever, the development of the eukaryotic cell possibly took as long as 1 billion
years before it resembled anything living today. Eukaryotic cells are typically
10,000 times larger than prokaryotes.
During cell division, DNA in the nucleus and in the organelles repli-
cated, with half the genes remaining with the parent and the other half passed
on to the daughter cell.This process, called mitosis, increased the likelihood of
genetic variation. It greatly accelerated the rate of evolution as organisms
encountered new environments and could adapt.The extraordinary variety of
plant and animal life that has arisen on this planet over the last 600 million
years is due exclusively to the introduction of the eukaryotic cell and its huge
potential for genetic diversity.
Early single-cell animals called protists were the progenitors of all other
animal species.They were the first group of organisms to evolve a nucleus and
shared many characteristics with plants. The cells contained elongated struc-
tures of mitochondria, which were bacterialike bodies that produce energy by
oxidation.They also contained chloroplasts, which were packets of chlorophyll
that provide energy by photosynthesis.
Many protozoans secreted a tiny shell composed of calcium carbonate.
When the animals died, their shells sank to the bottom of the ocean like a
constant rain. Over time, they built up impressive formations of limestone
(Fig. 18). The shifting of these sediments by storms and undersea currents
buried dead marine organisms that were not eaten by scavengers. A calcite
ooze was then formed,which eventually hardened into limestone, preserving
trapped species for all time.
Some varieties of protists formed large colonies, whereas most lived
independently.The entire body was composed of a single cell containing liv-
ing protoplasm enclosed within a membrane. Present-day single-celled organ-
isms have not changed significantly from ancient fossils. However, most archaic
forms were soft bodied and did not fossilize well. They obtained energy by
ingesting food particles or by photosynthesis and reproduced by using unicel-
lular gametes that do not form embryos.
The major groups of protists include algae, diatoms, dinoflagellates,
fusulinids, radiolarians, and foraminiferans (Fig. 19). The foraminiferans were
microscopic protozoans whose skeletons composed of calcium carbonate pre-
served much of the record of the behaviour of the ocean and climate. Most
lived on the bottom of shallow seas.A few floating forms existed as well.Their
remains are found in both shallow- and deep-water deposits. The fusulinids
were large, complex protozoans that resembled grains of wheat, ranging from
microscopic size to giants up to 3 inches in length.
The earliest protists were microorganisms that built stromatolite struc-
tures.Ancestors of blue-green algae built these concentrically layered mounds,
resembling cabbage heads, by cementing sediment grains together using aHistorical Geology