Humans Likely Caused Mammals to Shrink, Study Reports

Ancient humans may be the reason mammals have significantly shrunk in size over the past 125,000 years, according to a new study in the journal Science. This research comes from scientists at various U.S. universities, who discovered the trend by analyzing fossils going back 65 million years. They found that size did not play a role in mammalian survival until humans came about, Tech Times reports.

While large and small mammals both thrived for a while, humans rapidly shifted that balance. Data showed that when humans settled in a region the chances of survival for large mammals significantly dropped. In fact, as humans moved out of Africa large animals often went extinct wherever they went. That same trend showed up in the Americas as well. 

“The only time we see a spike in extinction rates and this huge size bias, where large-bodied things are disproportionately at risk, was where hominids are involved,” said lead author Felisa Smith, a researcher at the University of New Mexico, according to Seeker. “From that, we conclude that it’s probably related to human exploitation of large-bodied animals.”

The team found that as humans spread general animal size declined. The average body mass of mammals in Eurasia dropped by half over the course of 100,000 years, and species in Australia lost 90 percent of their mass over the last 125,000 years.

That is likely the result of hunting, but it is also possible that many larger mammals were killed because early humans feared them and saw them as threats. In addition, the burning of forests and grassland may have led to that shift.

“From a life-history standpoint, it makes some sense,” said study co-author Kate Lyons, a researcher at the University of Nebraska-Lincoln, according to USA Today. “If you kill a rabbit, you’re going to feed your family for a night. If you can kill a large mammal, you’re going to feed your village.”

Such populations drops did only affect the species being killed, they also had a large impact on the environment. Researchers found that killing off large mammals caused the land to erode faster as well. In that way, further of this topic could help scientists better understand the different ways our ancestors shaped the world of today.