Evolution can be a heads or tails question, with scientists debating which parts of animals diversified first. It turns out that heads win, according to new research, with species evolving in their heads before other bodily changes become evident.
The findings suggest that food availability has been a primary driver of animal evolution, starting with the head and then on down.
"Species evolved to exploit new food sources before shifting into new habitats or evolving new ways to get around," said Lauren Cole Sallan, lead author of the study published in the latest Proceedings of the Royal Society B.
"Strange heads show up first — crushing jaws, animals with big teeth, with long jaws — but they're all pretty much attached to the same body," added Sallan, who is a graduate student in the Department of Organismal Biology and Anatomy at the University of Chicago.
She and co-author Matt Friedman of the University of Oxford set out to test models that attempt to explain how adaptive radiations occur within the animal kingdom. For example, after a major disruption, such as an extinction event, surviving species diversify into myriad forms.
One popular theory, the "early burst model," holds that there's a flurry of divergence followed by a long period of relative stability. Another argues that habitat-driven changes in body type precede diversification of head types.
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To help resolve which is right, Sallan and Friedman analyzed two different adaptive radiations in the fossil record. The first was the explosion of ray-finned fishes after what's known as the Hangenberg extinction, an event 360 million years ago that decimated ocean life on Earth.
The second group was the acanthomorphs, a group of fish that diversified wildly around the time of the end-Cretaceous extinction that killed off non-avian dinosaurs.
For both groups, the researchers quantified differences in features such as body depth, fin position and jaw shape between species. They separated head features from body features in their study, to find out which changes actually occurred first and seemed to dominate.
Both sets of analyses found that diversification in head features came before diversification in body types. Earlier research on mammals, lungfish, birds and other animals suggests that these too followed the head-first pattern of evolution.
"An animal's first job is to obtain enough energy to live;, however a main tenet of natural selection is that there isn't enough of any one food in a habitat to support everyone past a certain population size," explained Sallan. "So there are two choices to get ahead: eat other foods in the same place, or move away and hope to find a habitat with the same food. In many cases, changing to a new food is probably simpler."
More evidence is needed to see if humans also fit into the head-first model of evolution.
Sallan pointed out that there has been some debate about whether or not walking upright, which stemmed from a change in habitat, happened before increases in human brain size. Feeding wasn't brought much into that theory, but could have also been a primary force upon the evolution of our species and hominid ancestors.
Luke Harmon, an assistant professor of Biological Sciences at the University of Idaho, was interested to learn about the fossil evidence.
"We have seen this (head-first) pattern in data from a wide range (of) living species, so it is very interesting to see it in fossil data from deep in the phylogenetic tree of vertebrates."