The geological history of biodiversity is about 3.5 to 4 billion years old (Table 7.2).
The first appearance of multi-cellular organisms was perhaps a milestone in the history of biodiversity which, did not diversify until about 600 million years ago (Mya).
In fact, by the end of the Cambrian we find that almost all of today’s major animal phyla are present in the fossil record.
Broadly speaking there were relatively few species during the Paleozoic and Mesozoic eras. But about 100 Mya there was a tremendous increase in biodiversity that culminated at the end of the Tertiary and beginning of the Quaternary (Pleistocene). During these periods we had more living species and higher taxa of plants and animals than at any time before or afterwards.
During the entire history of biodiversity we find dramatic increases as well as mass extinctions in certain periods (Seposke 1992). Thus, we find clear patterns of adaptive radiation, stabilization and extinction for both the terrestrial and marine groups. We find an overall increase in diversity through time, but we do not know the answer as to why the temporal diversification took place.
We have also noticed several differential patterns of diversification—most animals are insects (> 80%), more species of mammals are rodents (> 40%) and most species of plants are angiosperms (> 75%). Such ‘ disproportionate diversifications are not easy to explain. Some scientists believe that phytophagy ‘(plant eating) led to disproportionate diversification in insect groups (Mitter et al 1988). We are ‘ living in the Quaternary (Holocene) in a time of decreasing biodiversity, which is correlated with ‘ changes in climate and large – scale human activities.
The present scenario of biodiversity of different groups possibly indicates that the numbers of described species may not be accurate. The reliability of accuracy is good for plants and chordates; it is moderate for insects and poor or very poor for some other groups like nematodes, bacteria and viruses. It is possible that many species became extinct even before they were described by taxonomists and the present state is what we described as biodiversity crisis.