In all of Earth’s history, which species of mammal survived for the longest time?
Benjamin Burger, a paleontologist at Utah State University, has come up with two candidates that could not be more different: a tiny shrewlike animal that ate insects and a giant relative of the elephant that had a long snout and weighed more than four tons. Both lasted for at least 23 million years in the period after the dinosaurs died out but before modern humans appeared.
When Burger lowered the bar of survival to 20 million years, he found 19 types of mammals that lasted that long. The list included whale ancestors, marsupials, rodents, insectivores and a relative of the horse.
All of these are what scientists call exceptional survivors; most mammals are mere blips in the fossil record. They appear and evolve quickly, in evolutionary terms, and they go extinct quickly: The average duration is just 1.7 million years. Studying the tenacious holdouts may help scientists figure out if there are any traits that enable extreme longevity. And, conversely, researchers may be able to pinpoint traits that have hastened extinction and use that information to identify modern species that are at risk.
A faster tempo
This is particularly relevant now because many scientists believe the world is in the middle of a major mass extinction event, in which species are dying out faster than new ones are evolving. The Earth has experienced five such events; the crescendo of extinctions this time around is driven almost entirely by the changes wrought by human activity, our burgeoning populations and consumption of natural resources.
“We’ve accelerated the tempo of extinctions so much as humans that it is pretty astounding for people to think that some species survived for over 12 million years,” said Susumu Tomiya, a paleontologist at the Field Museum of Natural History in Chicago.
The elephant ancestor, Gomphotherium angustidens, appeared in the fossil record 23 million years ago. It was at least 9 feet tall, about the same as modern-day elephants. An opportunistic browser that thrived on savannas, it had four tusks and a long snout, which presumably helped it procure its plant-based diet. The species went extinct 12,000 years ago, just as modern humans were beginning to discover agriculture. Overall, it survived for 23 million years, coming in second place after the shrew.
Burger wanted to know whether Centetodon and Gomphotherium had traits in common with each other and with 103 other species that survived for more than 10 million years. He first theorized that heftier animals would be less likely to persist than small ones.
After all, a larger animal usually gives birth to fewer offspring, which can put the species at risk if the population does not get replenished quickly after a crisis. Large mammals are also more likely to be hunted. At the end of the last Ice Age 9,000 to 13,000 years ago, for example, mammoths and mastodons went extinct in part because of hunting by humans.
But Burger found that a large body did not affect longevity of a species.
“Some really long-lived mammals were fairly large,” he said. Case in point: Gomphotherium.
Is that so?
The role of size
Other scientists have reached different conclusions. A group led by Lee Hsiang Liow, an ecologist at the University of Oslo, examined mammal genera in Europe and western Asia that lived between 23 million and 2.5 million years ago. They found that smaller mammals were overall more likely to survive than larger ones. The small ones could hide in tree holes or burrows, or go into hibernation during times of chaos, Liow suggested in a study published in 2008 in the Proceedings of the National Academy of Sciences.
But that finding did not hold up when Tomiya examined North American mammals that lived around the same time. He found no difference in the longevity of large and small mammal genera in North America. In a study published in December in the American Naturalist, he found that both types had comparable duration.
That suggests something more complex than size was at play, Tomiya said. He thinks survival, then as now, depends on the interplay between environmental change and how well a species can cope with it by having the right biological features. Environmental changes may affect small or larger areas, quickly or extremely slowly. They may arrive without warning, such as asteroid impacts that many theorize killed off the dinosaurs, or be gradual, such as climate change or the arrival of a new predator species in a region.
A large body might be helpful during some changes, while diet or the geographical distribution of the animal might prove crucial during others. If the species does not have the right trait for its particular discomfort, its population will plunge.
The death knell – extinction – occurs randomly. A small population is especially vulnerable to disasters, and the nature and timing of the final blow has an element of chance.
When Burger looked at species that lived on various continents from the Mesozoic period (160 million years ago) to today and tried to figure out whether certain traits were more important for survival over others, time appeared to wield an indiscriminate axe.
The shrew, the elephant ancestor and other mammals that he identified had little in common. Some were big; others were small. They existed at different times. They had different diets; they lived in various ecosystems. There was no single reason why they survived any longer than their contemporaries, Burger said.
“The equivalent would be sort of the Volkswagen Bug – those types of models of mammals that just stick around, they become sort of classic,” Burger said. “For whatever reason, whatever they had seemed to work really well, and they didn't need to change.”
The right traits
Every species (including humans and cockroaches) is trying to survive for as long as it can. But only some have the right biological traits to hang on for millions of years.
Burger has calculated the probability that Homo sapiens will become one of these long-surviving species. We have a 1 percent chance of lasting for 10 million years, he figures, and a 0.2 percent chance of hitting 20 million years.
“These are not good odds, but given that humans have been around as a species for around 250,000 years, we still have many more years before we find our species extinct,” he said.
Burger considers it unlikely that climate change will lead to the extinction of the human race. It would take a much more cataclysmic event to wipe out all 7 billion people on the planet.
“Climate change is bad, but it will likely not wipe out humans,” he said. “Humans were here roaming the Ice Age world, when (what is now) New York City was buried under 1,000 feet of ice as glaciers spilled out to form Long Island and Queens. Humans survive these periods of climate change. We have witnessed this change over the last 250,000 years. We can move. Life may not be good in the much hotter future, but humans, I’m confident, will survive, even with much-limited numbers.”