Can we discern a relationship between the complexity of life and its risk of succumbing to mass extinction? Recent evidence suggests that as organisms become more complex, their risk of undergoing extinction increases. As an organism's complexity increases, so too does its fragility, hence increasing complexity in most cases narrows the environmental tolerances of a living organism. Any bacterium can withstand the rigors of outer space (at least for a short period of time), but no animal can. As we move from bacterial life forms to protozoa and then to metazoans, the range of temperature, food supply, and environmental chemistry in which life can persist becomes more restricted.
This generalization seems to apply not only to individuals of a species but also to the species itself. One of the strongest messages communicated by the fossil record is that extinction rate is a function of complexity. On average, simple animals are far more successful at avoiding extinction and thus persist (in terms of geological time) far longer than complex ones, the simpler the species, the longer its reign on Earth. Many bacterial fossils found in 3-billion-year-old rocks are identical to living forms found commonly on Earth today. Are they the same species? Unless we can compare the DNA content of the ancient form with that of its living analog, we cannot tell. But our best guess is that they may indeed be the same species,- they certainly have the same external morphology. Simple bacterial species, once evolved, seem to last a long time, perhaps because of their very simplicity and ability to adapt without resorting to new body forms. Complex metazoans, on the other hand, show far shorter ranges, and even among metazoans the inverse correlation between complexity and evolutionary longevity seems to hold. For example, mammals (the most complex animals on the planet) have average species longevity only slightly greater than a million years, whereas bivalve mollusks, which are far simpler, last an order of magnitude longer.
But how can complexity be measured? Perhaps a bacterium really is no simpler than a complex metazoan, and the relationship purportedly observed is due to chance or something other than complexity. It turns out that there are ways of comparing complexity. Determining the length of the genome (and the number of genes it includes) is one such way, and an even easier method of characterizing complexity in a metazoan is by describing the number of different cell types it includes, as initially suggested by paleontologist James Valentine.
Zoologists and physiologists have carried out the differentiation of cell types in animals for years. A bacterium or a paramecium has only a single cell, of course, but with the advent of animal life exhibiting multicellularity, various body cells became specialized. A sponge, among the simplest of multi-cellular animals, has at least four cell types: one for catching food, one for secreting spicules (primitive supporting structures), one for transporting material around the body, and one that acts as a type of skin cell. Vertebrate animals such as ourselves have many more than this; mammals have in excess of 100 cell types.
Oddly enough, no one has yet tried to correlate complexity as measured by number of cell types with evolutionary longevity. The latter, also known as extinction rate, has been calculated for most groups of animals and plants by paleontologist Jack Sepkoski. Here we have combined these two sets of data and searched for correlation. The results seem to substantiate the view that complexity comes at a price of lower evolutionary longevity. This finding suggests that ever more complex animal or plant species show ever shorter evolutionary ranges, whether on Earth or elsewhere. And it suggests that risk of extinction increases through time (see Figure 8.4).
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