Mammalian Evolution Mirrors Climate Change
A recent study by an international group of evolutionary biologists has pointed to six broad yet distinct ℠waves´ of climate-induced mammalian diversity in the last 65 million years of evolution. Researchers say that extended periods of warming and cooling appear to signal the shift from one dominant grouping of mammals to the next.
In the online version of the journal The Proceedings of the National Academy of Sciences (PNAS), Professor Christine Janis of Brown University and a handful of Spanish colleagues from the University of Malaga and the University of Valencia describe six distinct groupings of species diversity, or “evolutionary faunas”. According to the researchers´ data, the emergence of four of these distinct faunas bore statistically significant correlations with large changes in the earth´s climate while the remaining two faunas appear to be more closely associated to large-scale invasions of foreign mammals from another continent than to climate patterns.
“Although we´ve always known in a general way that mammals respond to climatic change over time, there has been controversy as to whether this can be demonstrated in a quantitative fashion,” explained Professor Janis, the study´s senior author.
“We show that the rise and fall of these faunas is indeed correlated with climatic change–the rise and fall of global paleotemperatures–and also influenced by other more local perturbations such as immigration events.”
Whereas earlier evolutionary studies examining the relationship between animal species and drifting climatic conditions have focused on overall species diversity, for this analysis the biologists looked instead for distinct patterns within the various species.
This novel approach led to the observation of six distinct and consecutive waves of mammal species. Within each wave, the various species shared a common rise, crest and decline in their respective numbers, with the decline of one wave of mammals signaling the rise of the next. For instance, some 50 million years ago the fossil record shows that the so-called “Paleocene fauna” was at the tail end of its decline while the “early-middle Eocene fauna” was the up-and-coming dominant wave of mammal species.
Of particular interest was the fact that these shifts in dominance from one fauna wave to the next corresponded with large scale temperature changes. The researchers also noted shifts in overall species diversity using the other method.
By recognizing the relationship between changing patterns of weather and the disappearance or emergence of entire groups of mammals, researchers are also beginning to be able to make sense of specific characteristic traits seen in the various fauna-waves. For instance, when the Miocene epoch saw average global temperatures rising some 20 million years ago, grasslands began to overtake woodlands as the dominant form of vegetation, leading to a dominant group of herbivorous mammals with high-crowned teeth for chewing grasses.
The researchers were quick to stress that although these new insights are tremendously helpful in reconstructing and understanding life´s fascinating evolutionary narrative, they do not help us predict evolution´s future trajectory or the specific impact that current climate change may have on the world´s current species.
“Such perturbations, related to anthropogenic climate change, are currently challenging the fauna of the world today, emphasizing the importance of the fossil record for our understanding of how past events affected the history of faunal diversification and extinction, and hence how future climactic changes may continue to influence life on earth,” wrote the paper´s authors.
Image Caption: This painting by artist Carl Buell depcits a scene from the late Eocene of North America. The rhino-like animals in the background are brontotheres. The pony-sized Hyracodon, a closer relative of living rhinos, in the foreground. Credit: Courtesy of Carl Buell
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