The spread of hominims - early humans and relatives such as Neanderthals - from Africa coincided with the extinction of mammals such as the mammoths, sabre-toothed tiger and glyptodon, an armadillo-like creature the size of a car.
"There is a very clear pattern of size-biased extinction that follows the migration of hominims out of Africa," lead author Felisa Smith of the University of New Mexico told Reuters of the study published in the journal Science.
Humans apparently targeted big species for meat, while smaller creatures such as rodents escaped, according the report examining trends over 125,000 years.
In North America, for instance, the mean body mass of land-based mammals has shrunk to 7.6 kg (17 lbs) from 98.0 after humans arrived.
If the trend continues "the largest mammal on Earth in a few hundred years may well be a domestic cow at about 900 kg" (2,000 lbs), the U.S. team wrote.
That would mean the loss of creatures including elephants, giraffes and hippos. In March, the world's last male northern white rhino died in Kenya.
But other research casts doubt on a continued shrinking of mammals, partly because of conservation efforts to stave off threats to wildlife such as climate change, loss of forest habitats and pollution and expanding cities.
Thomas Brooks, chief scientist of the International Union for Conservation of Nature (IUCN) who was not involved in the study, said the projection of ever smaller mammals was "doom and gloom."
And a Red List of threatened species, maintained by the IUCN, also lists some wild mammals roughly the size of a cow - such as the African buffalo or the brown bear - as unendangered.
The Science study also excludes marine mammals such as the blue whale, the largest creature that has ever existed. It is endangered on the Red List but the populations are rising after a moratorium on hunts.
Smith said that "my optimist hat would like to say that it's not going to happen because we love elephants'." But she said populations of large land mammals were falling and "declining population is the trajectory to extinction."
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.