Female fish need to produce so many eggs because of the reproductive strategy in these species results in the highest number surviving to adulthood.
Rather, the rate of predation on fish eggs is high because they produce such a lot of eggs.
Fish first appeared during the Cambrian Explosion Era around 530 million years ago.
The first fish were primitive jawless forms (agnathans) which appeared in the Early Cambrian, but remained generally rare until the Silurian and Devonian when they underwent a rapid evolution.
The Cambrian explosion, Cambrian radiation, Cambrian diversification, or The Biological Big Bang refers to an interval of time approximately 541 million years ago in the Cambrian Period when practically all major animal phyla started appearing in the fossil record.
Cambrian explosion, the unparalleled emergence of organisms between 541 million and approximately 530 million years ago at the beginning of the Cambrian Period.
The event was characterized by the appearance of many of the major phyla (between 20 and 35) that make up modern animal life.
The Cambrian Period marks an important point in the history of life on Earth; it is the time when most of the major groups of animals first appear in the fossil record.
This event is sometimes called the "Cambrian Explosion," because of the relatively short time over which this diversity of forms appears.
Today, scientists have several theories to explain the phenomenon.
One theory is that life may have been able to evolve quickly because the Earth had changed so much that it created enough new niches that species could rapidly fill them.
Given the importance of oxygen for animals, researchers suspected that a sudden increase in the gas to near-modern levels in the ocean could have spurred the Cambrian explosion.
It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla.
The event was accompanied by major diversification in other groups of organisms as well.