The diversity of life through time shows some striking patterns. For example, animals appear in the fossil record about 550 million years ago, in a huge burst of diversification called the Cambrian Explosion. Many groups of organisms seem to originate this way, but later in their evolutionary history, their indexes of diversification and morphological change seem to diminish.
This type of patterns can be seen both in the fossil record and in the reconstructions of past diversity by observing the relationships between living organisms, and has given rise to many questions such as: Do organisms have more evolutionary flexibility when they evolve for the first time? Or do ecosystems fill up, as more species evolve, giving fewer opportunities for further diversification later on?
In their new article, published in Evolution, experts Graham Budd and Richard Mann argue that these patterns can be largely illusory and that we would expect to see them even if evolutionary rates of change remained stable over time.
Biologists and paleontologists use statistical models called “models of birth and death” to study the way in which random events of speciation and extinction give rise to patterns of diversity. Just as one can roll a die five times and get five sixes or none, the results of these random models are highly variable.
These statistical fluctuations are particularly important at the origin of a group when there are few species. It turns out that the only groups that survive this initial period are those that are rapidly diversifying, all others are extinguished.
How exactly are these groups that continue to be the great successful groups that we see living today, and that fill most of the fossil record, it follows that they are likely to show this rapid pattern of diversification at their source, but only because they are a biased subset of all the groups. Later in its history, when such groups are diverse, the statistical fluctuations have much less effect and, therefore, their rate of evolution seems to be reduced to the background average.
As a result, the patterns we discover through the analysis of such groups are not general characteristics of evolution as a whole, but represent a remarkable bias that emerges only by studying groups that we already know were successful. This bias, called “the push of the past”, has in fact been known theoretically for about 25 years, but has been almost completely ignored, probably because it was supposed to be of negligible size.
However, Budd and Mann show that the effect is very large, and in fact can explain much of the variation we see in past diversity, especially when combined with the effects of large “mass extinctions” like the one that killed out of the dinosaurs about 66 million years ago said the authors.
The push from the past is an example of a much more general type of pattern called “survival bias” that can be seen in many other areas of life, for example in start-ups and finance, and in the study of history . In all these cases, the failure to recognize the bias can lead to highly misleading conclusions.
For Budd and Mann, the history of life itself is not immune to such effects, and that many traditional explanations of why diversity changes over time may need to be reconsidered, a point of view that will surely prove controversial.
All the major groups of animals appear in the fossil record for the first time around 540-500 million years ago. However, another work published in Proceedings of the National Academy of Sciences (PNAS) suggests that for most animals it was not an explosion , but a gradual process . The new analysis presents a challenge to the two main competitive hypotheses about early animal evolution.
The Euarthropoda are the largest group of animals on Earth and contain insects, crustaceans, spiders, trilobites and a wide variety of other forms of living and extinct animals. They comprise more than 80% of the animal species of the planet and are key components of all the ecosystems of the Earth for more than 500 million years.
Now, an exhaustive analysis of this great group, conducted by the universities of Oxford (United Kingdom) and Lausanne (Switzerland), shows that it evolved gradually , an idea that challenges the main theories of early animal evolution.