Why we don’t have fins: when humans split from watery kin
Even the hotties and hunks of the human race would likely have oblong-shaped bodies, great rolls of blubber and sleek bullet heads if Homo sapiens had adapted to the ocean like aquatic mammals did eons ago, Stanford University researchers said Monday.
That’s because certain changes, including insulating layers of fat and a fish-like physique, would have been necessary to keep the body warm and moving fast enough to find food in the cold water. It means the real-life version of Aquaman would weigh close to 1,000 pounds, said Jonathan Payne, a Stanford professor of geological sciences.
“I would expect that a hominin that began evolving an aquatic lifestyle would need to acquire a decent layer of blubber for insulation, would not need large ears, and would likely modify its limbs to improve locomotion in water,” Payne said. “It appears quite likely that the selective pressures would initially favor size increase.”
His research, published Monday in the Proceedings of the National Academy of Sciences, found that oceangoing mammals, including seals, sea lions, manatees, dolphins and whales, gravitated toward an ideal, ample weight and made adaptations to improve their insulation or feeding ability after their ancestors entered
water millions of years ago.
“We found that living in the water requires a minimum size so that heat loss isn’t so dramatic,” Payne said. “About 1,000 pounds allows for an energy surplus, and that’s what you want to be a successful species. That appears to be the size at which its easiest to be a marine mammal.”
The study started as an attempt to learn why most of the largest creatures in the sea — blue whales, orcas, walruses and elephant seals — are mammals. Payne and graduate student Will Gearty, the report’s lead author, joined with Craig McClain of the Louisiana Universities Marine Consortium to compile the body masses of 3,859 living species and 2,999 fossil species, 25 percent of them extinct, and analyze how they changed over time.
The researchers followed the ancestral histories of seals and sea lions, which are closely related to dogs; manatees, which share ancestry with elephants; and whales and dolphins, which are related to hippos and hoofed land mammals. They found that the primary rule governing the size and shape of seagoing creatures is the need to stay warm in cold water and maintain enough energy to find food.
The study of aquatic mammals tells a story about evolution after the extinction of the dinosaurs 65 million years ago. Today’s aquatic mammals began as terrestrial animals. They were almost certainly semiaquatic — wading and swimming along the coast — but could not fully commit to the sea because they would have been eaten by huge reptiles and giant squid-like cephalopods.
Once these carnivores were killed off, the mammals began entering the water. The whale, Payne said, evolved over the next 10 million years from a partially adapted swimming mammal to a fully aquatic species. Bones that were once legs still exist in whale skeletons.
Payne said humans’ primate lineage goes back to about the time the ancestral whales entered the water. But as aquatic mammals diversified, many of them grew larger than their closest land-dwelling relatives. In fact, most of the reptile and arthropod species that entered the water after the dinosaur extinction also grew to be larger than their land-based cousins.
The general consensus has been that the buoyancy of living in water removed the need to fight gravity and enabled unlimited growth as long as animals found enough to eat, but Payne and his colleagues found that wasn’t true. Their hypothesis is that, at some point in the evolutionary process, the water constrained growth and pushed aquatic mammals toward a more uniform size.
“The key thing is that when you evolve to become a fully aquatic animal, the range of sizes appears to be smaller than land animals,” Payne said.
The researchers concluded that land animals evolved quickly once they took to the water, bulking up if they started out small and trimming down if they were large, but not always at the same pace. For instance, the dog relatives grew faster than the hippo relatives shrunk, suggesting that larger, more blubbery animals have a slight evolutionary advantage in aquatic environments.
“It’s not that water allows you to be a big mammal,” Payne said. “It’s that you have to be a big mammal in water — you don’t have any other options.”
Still, all of the creatures tended to converge up or down toward the 1,000-pound sweet spot, an example of how different species evolve shared traits when they live in similar environments. The exceptions to the 1,000-pound rule were whales on the large side and sea otters on the smaller end. That’s because they evolved defenses against the harsh realities of the sea, according to the study.
Blue whales, the largest animals that ever lived, can strain massive amounts of food from seawater through their baleen filters, allowing them to expend less energy hunting and thus grow larger than their toothed counterparts. Sea otters, on the other hand, retain more heat because of their dense fur, which has the unique ability to trap air bubbles and insulate the body from the cold.
Although the study doesn’t address human evolution, the results could alter our understanding of Homo sapien history. Some scientists have hypothesized that humans were once on the cusp of being aquatic.
In 1960, marine biologist Alister Hardy suggested a branch of human ancestors may have been forced by competition to leave the trees and live off shellfish by the sea shore. The theory, called the “aquatic ape hypothesis” — which has been mostly derided by mainstream academics — could explain human characteristics like upright posture, hairlessness and the regression of the olfactory organ.