About 13,000 years ago, North America had a more diverse mammal community than today's Africa. There were several types of horses, camels, llamas and an extinct animal called Glyptodon that looked a bit like a Volkswagen belted armadillo. Smilodon a saber-toothed cat the size of today's African lion, sneaked through the prairie in search of sloths and mammoths. Seven foot long giant otters hit massive trees. And such giant creatures were not found only in North America. On all continents, the mammals in the late Pleistocene, the geological epoch of about 2.5 million to about 11,700 years, were much larger on average.
Scientists have long debated why all these large-scale living things are dying out, while many of them have died out, their smaller counterparts have survived. A research team led by biologist Felisa Smith of the University of New Mexico analyzed evidence of millions of years of mammal extinction and found that large mammals on each continent emerged at the same time humans first appeared. They announced their results on Thursday in Science.
If the extinction trend continues, modern elephants, rhinos, giraffes, hippos, bison, tigers, and many other large mammals will soon disappear, as humanity's main threats have spread to indoctrination, poaching or other forms of killing Include processes such as habitat loss and fragmentation. The largest terrestrial mammal in 200 years could well be the domestic cow, suggest Smith's research findings.
Some scientists put the blame of humanity on their shoulders, arguing that the planet's megafauna must be overrun. After our hominid relative Homo erectus emerged from Africa in Eurasia about two million years ago, Homo sapiens followed about 60,000 to 80,000 years ago and spread in Eurasia to our close cousins, the Neanderthals and Denisovans. It is thought H. Sapiens later reached Australia between 50,000 and 60,000 years ago and eventually settled in America 1
In their new study, Smith and her team have created a database of all land mammals that lived 65 million years ago. They divided this timeline into one-million-year pieces and analyzed the extinction trends for each of them. "We have found absolutely no impact of the climate on the extinction of mammals over 65 million years," she says.
But about 125,000 years ago, and until today, large mammals are more extinct than smaller researchers found. The average size of the surviving mammals has thereby decreased. And these exterminations of large mammals are closely linked to the appearance of humans.
In North America, the average mammal weighed about 98 kilograms before the ancestors of humans emerged. Today, the average size is closer to eight kilograms. "We've reduced the distribution of mammals by a few orders of magnitude [body sizes]," says Smith. For most mammalian evolutionary stories, the size of an animal was unpredictable for its risk of extinction. This connection did not appear until hominids began to live with large mammals.
This result does not mean that climatic changes could not affect some wild animal populations, so people could more easily bring about their possible demise. Rather, it suggests that the likelihood that large mammals will die out is associated with human activities. A series of animals that had emerged in Eurasia, Australia, and America without the risk of predators from tool-use, fire-making, and group living hominids suddenly faced a new threat. They just could not adapt fast enough to survive the invasion of these omnivorous bipedal monkeys.
In addition, Smith investigated the size distribution of African mammals prior to hominid migration to Eurasia. She found that on average, African mammals were smaller as soon as hominids showed up in the landscape – and they developed right next to each other. "They have evidence that hominids in Africa have already affected the size distribution of mammals on this continent before Homo sapiens emerged," says paleozoologist Emily Lindsey, deputy curator and excavation director of the La Brea Tar Pits Museum in Los Angeles, who was not involved in the study. What that means, she says, is "these groups of hominid species have an impact on a continental scale before the evolution of modern man." And it does not take so many hominids to have such broad implications. Taking a large species from extinction does not mean killing every one of its members. "You just have to kill a little more than every year you produce," says Lindsey. If a population's reproduction rate can not balance its losses every year, the species will simply die out within a few hundred to a few thousand years.
Large mammals are particularly susceptible because they multiply slowly. For example, mammoths and mastodons probably had a two-year gestation period, similar to modern elephants, and would typically have produced only one offspring at a time. It is therefore much easier to decimate a population of 100,000 mammoths than a population of 100,000 rabbits multiplying twice a year and giving birth by dropping.
Massive animals have a disproportionate impact on their ecosystems. They disperse seeds, cut down trees and compact the soil with each step. The shape of the paths that they incorporate into the hills influences the flow of water and erosion. Large animals also create habitats for smaller creatures. For example, elephant footprints create important habitats for pondlet invertebrates.
Smith says the lesson to be learned from the new insights is that our hominid inheritance has made us extremely skilled killers. "What's different now," she says, "is that some of us feel comfortable, have a sufficiently high standard of living, that we can begin to think about our use of the earth." Rather than acting as a consumer, many of us are now able to become environmental officers.