For millions of years the extinction rates among large, medium and small land mammals were similar. Yet the large species started dying off much faster, about 100,000 years ago in Eurasia, 50,000 years ago in Australia, and 15,000 years ago in North and South America A. These shifts, it turns out, correspond with when a hominin species—Homo erectus, Homo neanderthalensis and especially Homo sapiens—spreads across a continent. “There is an astoundingly tight fit” among the data sets, says Felisa A. Smith, a paleoecologist at the University of New Mexico, who led the research. Hefty animals suffered from being hunted, as well as habitat change and fires caused by human activities. The imbalance continues today, leaving far fewer massive animals, even though small ones go extinct, too. Two centuries from now, cows may top the size chart B. “We have changed the entire Earth,” Smith says. “Now we have to be nature’s stewards.”

Credit: Lucy Reading-Ikkanda; Source: “Body Size Downgrading of Mammals over the Late Quaternary,” By Felisa A. Smith et al., in Science, Vol. 360; April 20, 2018