3.5 billion to 700 million years ago


Toward the end of the Precambrian, eukaryotic cells had evolved, these cells were increasing their variation through sexual reproduction, oxygen was accumulating in the atmosphere, and the climate had stabilized after severe ice ages. In the aftermath of these changes, eukaryotic cells began to form small multicellular agregates which became the first animals.

Sexual reproduction seems to have occurred since 900 million years ago. Not all reproduction involves sex. Bacteria can't have sex although genetic material can be exchanged between organisms through several processes. Protists, plants, fungi, and even animals can reproduce without sex. This asexual reproduction simply involves making a duplicate cell of the existing organism that develops into a new individual. Asexual reproduction in unicellular organisms simply involves a cell splitting in half. Sexual reproduction involves the fusion of 2 specialized cells called gametes (typically from two separate individuals). In sexual reproduction, the formation of gametes requires a process (meiosis) which produces cells with only half the normal number of chromosomes; this cell division differs from mitosis in which a cell simply copies itself.
Why sex? Asexual reproduction has many advantages: each individual can reproduce (the population grows much more quickly than sexual populations), immigrants to a new area can populate it instead of being celibate for life, and individuals can pass on their good genes to their offspring without the possibility of the mate providing a horrible set of genes. Sexual reproduction not only lacks in the above aspects, in higher organisms it also leads to enormous energy output and even life-threatening sacrifices.
The only advantage to sexual reproduction that compensates for all of these disadvantages is that it produces variability in offspring. Variable offspring are more likely to have at least some individuals that are resistant to new parasites or environmental changes. After the advent of sexual reproduction and the ability to produce such variable offspring, evolution occurred at a rapid pace compared to the more than two billion years of very little change.
Toward the end of the Proterozoic Eon, eukaryotic cells had evolved and they could reproduce sexually. Atmospheric oxygen levels increased, in part produced by changes in the processing of marine organic matter (Logan, 1995; Knoll, 1996). Finally, the climate stabilized.

In the late Precambrian, the earth experienced a number of great changes. The landmasses, which had been united in a supercontinent known as Rodina, began to separate. A number of ice ages occurred. Precambrian glacial deposits have been found on every continent except Antarctica. Separate glaciations seem to have occurred 850 million years ago (plus/minus 50 million years), 740 million years ago (plus/minus 20 million years), and 650 million years ago (20 million years (Fedonkin, 1990; Kerr, 1998; Muller, 2001). At its coldest, the temperature of the earth may have dropped to -50oC. Such frigid global temperatures would have frozen all landmasses and the oceans might have been covered with ice sheets. Life may have been restricted to hot springs and other areas of geothermal activity and to meltwater pools on the surface of the ice.
Carbon isotope data suggests that the biological activity in the oceans prior to this glaciation virtually ceased. These glaciations may have caused the extinctions of microscopic protist-like organisms referred to as acritarchs (Chuanming, 2001). Without evaporation from the oceans and the resultant rain, carbon dioxide from volcanic activity would have accumulated unchecked. Carbon dioxide levels would eventually greatly exceed modern concentrations (maybe by a thousand times) and this would lead to the warming of earth. This warming might have been quite extreme, perhaps to 50 degrees Celsius (Hoffman, 1998; Minnerop, 2000; Knoll, 1986).
At the end of the Precambrian, the Pacific edges of North America, South America, Australia, and Antarctica were formed. The western part of North America seems to have been joined to Antarctica at that time (Dalziel, 1985).
By the end of the Proterozoic, the climate had stabilized, oxygen had accumulated in the atmosphere, and eukaryotic cells existed which were increasing their variability through sexual reproduction. What came of this? The animals.