2.5 billion years’ of evolutionary history lost

The world has lost 2.5 billion years’ worth of unique evolutionary history in the past 130,000 years, say the authors of a new study on biodiversity that looks to quantify how damaging the current, sixth, mass extinction will be. 

It will take millions of years, much longer than humanity is expected to live, to recover the phylogenic diversity lost since the last interglacial period. 

The authors, led by Matt Davis of Aarhus University in Denmark, chose that timeframe because “it better represents the typical, megafauna-rich state that existed through much of the Cenozoic.” 

The Cenozoic period, which began 66 million years ago, is called the ‘Age of Mammals’. More than 300 mammal species have gone extinct in the past 130,000 years, report the authors in the journal PNAS.{%recommended 6202%}

The study looks specifically at phylogenetic diversity – branches on the evolutionary tree – as a barometer of the extinction crisis, as opposed to biodiversity, which is a simple count of different species. The latter, as the authors note, “implicitly treats all species equally”, whereas phylogenic diversity “is a complementary metric that measures lineage history and may be correlated to functional trait diversity and evolutionary potential”.

This approach means that extinction of the pygmy sloth (Bradypus pygmaeus), for instance, of which only about 500 remain, could be made up in less than two years by the other 5418 mammals still extant, because the pygmy sloth’s lineage split only 8900 years ago. The aardvark (Orycteropus afer), on the other hand, represents 75 million years of evolution, because it is the last remaining species of its order. 

“Biodiversity is more than the number of species on Earth,” Davis and colleagues write. 

“It is also the amount of unique evolutionary history in the tree of life. Such deep cuts into the mammal tree are increasingly likely, given that over one-fifth of current mammal species are threatened with extinction.”

Some of that extinction can be directly linked to human activity. For instance, a paper published in April 2018 found a link between human migration and the extinction of megafauna. Anthropogenic climate change, hunting, and habitat depletion are all examples of how humans contribute to animal extinction. 

The debate is over and we are now “emphatically” in a period of great extinction, according to Bill Laurance, of James Cook University, and Paul Ehrlich, president of the Centre for Conservation Biology, who combined to write a strongly worded article in 2017. 

“Without decisive action, we are likely to hack off vital limbs of the tree of life that could take millions of years to recover,” they state.