【I Am a Plaything】

Cows -- simple-minded,I Am a Plaything prone to belching, and eager to eat -- may be the planet's largest land animals in two or three centuries.

But it's not because they're getting any bigger.

Rather, these approximately 2,000-pound ungulates could be the largest land mammals left alive in the next few hundred years.

SEE ALSO: Rhino experts aren’t banking on unproven IVF technology to rescue threatened species

Ever since our human ancestors became interested in eating meat some 1.8 million years ago, the biggest animals have been expertly hunted, driving populations down.

In fact, spear-wielding hunters, not climate change, could be the defining reason for the steady demise of Earth's largest mammals, argue scientists in a study published Thursday in the journal Science.

The trend, they say, continues today.

"The only time being big is bad is when humans are involved," Felisa Smith, a professor of biology at the University of New Mexico and lead author of the study, said in an interview.

Today, elephants are the largest land animals on Earth. Credit: Ronesh Parbhoo / Barcroft Images / Barcroft Media via Getty Images

"We are efficient predators and have been for a really long time -- so there's not a value judgment here -- it's just what hominids did," said Smith.

There's an ingrained idea that being big, like a rhino or wooly mammoth, naturally predisposes a mammal to extinction, particularly during times when the climate substantially changes.

"But that's wrong," she said. "Shifts in climate influenced adaptions, but they didn’t drive extinction."

In the past, large mammals could avoid extinction by traveling elsewhere, to more suitable habitat. "They’ve dealt with climate before," Smith said. But with human development and the destruction of wilderness, that option has largely disappeared.

"Today the problem is we’ve cut off adaption," said Smith. "Big mammals are hemmed in by development and human influence."

Smith and her research team analyzed 65 million years of mammalian fossil data on each continent (excluding Antarctica), splitting this vast period into 1 million year intervals and assessing mammal diversity and extinctions at each time.

Separately, they zoomed in on the last 125,000 years, after a wave of humans had migrated through and left Africa.

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The impact of humans in Africa "was striking," said Smith.

Around 125,000 years ago, the average body mass of mammals on the continent was just half the average body mass of mammals in North and South America, where, critically, there were no humans at that time.

A member of the Bushmen, an indigenous African people, prepares to throw a spear at a winded gemsbok, which is surrounded by the hunter's dogs. Credit: Nat Farbman/The LIFE Picture Collection/Getty Images

Early humans, like the tool-using Homo erectusand the more recently extinct Neanderthals, likely wiped out large fauna on the wide African plains, a place one would expect large creatures to flourish.

"This suggests archaic human influences on mammal diversity, body size, and the number of mammals," said Smith.

The dramatic size difference between the animals on these continents "provides strong evidence" that early humans were largely responsible for depleting large mammals from these areas, Ben Campbell, a biological anthropologist at the University of Wisconsin-Milwaukee who was not involved in the study, said in an interview. Campbell did note that climate change could still have been an influential factor.

This idea, that our ancestors successfully hunted massive mammals, however, isn't too surprising.

We now know Neanderthals were more sophisticated than we once thought, and at the very least, they had stabbing spears, said Campbell. Early modern humans didn't yet have bows and arrows, but could have fashioned deadly throwing spears.

"Projectile technology makes you more effective," Campbell said.

What's more, unlike humans today, early humans lived in the wilds and developed a keen understanding of how to take down large, meaty creatures.

"They lived with these animals and they understood these animals," said Campbell. "We overvalue technology. It’s their knowledge."

Why the cow?

It's no secret that many of the largest land mammals on the planet are endangered, some critically so.

The last male northern white rhino, named Sudan, died last month. African elephants have been listed as endangered since 1978. Giraffe populations, long thought safe, showed "dramatic declines" between 1985 and 2015, and are now listed by the International Union for the Conservation of Nature (IUCN) as endangered.

Smith and her team found that if all the animals currently at risk for extinction, according to the IUCN, go extinct within 200 years, "the largest mammal on Earth in a few hundred years may well be a domestic cow," the researchers wrote.

Fatu and Najin, the last two remaining northern white rhinos, both females. Credit: Georgina Goodwin /Barcroft Media via Getty Images

Overall, this would mean that the average global weight of mammals would be the lowest in 45 million years, the study found.

Cows don't have even the remotest threat of going extinct. There are millions of cattle blanketing pasture land globally. Most of all, we work hard to keep them alive.

"The only reason why we’ll have something as big as a cow is because we like cows — they’re domesticated," said Smith.

Losing all threatened and endangered species over the next two or three centuries might sound like a worst-case scenario. But, according to the team's peer-reviewed results, which Campbell noted were done well, this is in line with a long-term reduction in mammal sizes and a decrease in biodiversity.

Smith acknowledges this is "a sad message," but she said it provides a reason for us to "do something about this if we care about biodiversity on the planet."

"Mammal extinctions are rarely synonymous with climate, but are always synonymous with human arrival," she said.

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