Key to Ants’ Evolution May Have Started With a Wasp
How should we judge the success of an animal? Philip S. Ward, a biologist at the University of California, Davis, offers what could be called the Picnic Test. “Have a picnic anywhere in the world,” he suggests. “Who would pick up the crumbs?”
Unless you happen to lay down your picnic blanket in Greenland, Antarctica, or a few remote islands in the Pacific, the answer will be ants. Ants have spread to just about every corner of earth’s dry land, colonizing virtually every imaginable ecosystem. By one rough estimate, there are 10,000 trillion ants on earth at any moment. In one study in a Brazilian rain forest, scientists discovered that the total mass of the ants that lived there was about four times greater than that of all the mammals, reptiles and amphibians combined.
One factor in the spectacular success of ants is their social life. They live in large colonies in which they divide the labor of finding food, rearing their young and defending their nests. Their societies are so complex that some scientists have studied ants as a way to understand the factors behind our own evolution into a social species.
It’s thus no surprise that many biologists — Dr. Ward among them — have long wondered how ants evolved. In the journal Current Biology, Dr. Ward and colleagues at the University of California, Davis, and the American Museum of Natural History, have now published an evolutionary tree of ants and their closest relatives that may provide the answer.
The authors conclude that the ancestors of ants were wasps. Not just any wasps, though: the closest relatives of ants turn out to include mud dauber wasps, which make pipe-shaped nests on the walls of buildings.
Scientists first started searching for the origin of ants by looking at their bodies. The oldest ant fossils are 100 million years old. But the fossils don’t offer a clear indication of what the forerunners of ants were like. Scientists were able to narrow the field by comparing ants with other living insects. They came to agree that ants were closely related to stinging wasps and bees.
Unfortunately, that’s a bit like saying you’re closely related to Europeans and Asians. This group of insects includes hundreds of thousands of species of wasps and bees to choose from. Different teams of researchers argued for different lineages as the closest relatives to ants. Some researchers argued, for example, that ants evolved from wasps that sting prey and then lay eggs on their victims.
In the 1990s, Dr. Ward and other researchers started sequencing genes from insects to gather more evidence, but for years a clear answer remained elusive. Recently, however, Dr. Ward and his colleagues took advantage of the explosion in DNA sequencing power to take a fresh look at the old problem of ants. “We decided to generate data anew,” Dr. Ward said.
Dr. Ward traveled around California and Nevada to catch ants, wasps and bees. He and his colleagues then sequenced more than 300 genes from one species of ant and 10 species of bees and wasps, each representing a different major lineage. In the end, they produced a database far larger than in previous studies.
“This work is meticulously done,” said Jacobus Boomsma of the University of Copenhagen, who was not involved in the study.
The scientists then used computers to search for an evolutionary tree that best accounted for their genetic data. Their research clearly pointed to mud dauber wasps and bees as the closest cousins to ants.
Next the scientists used statistical methods to determine how likely it was that their results were correct. The link between ants, mud dauber wasps and bees was strong. “The support of the tree branches is impressive,” said Dr. Boomsma.
The new study suggests that mud dauber wasps may offer clues to the evolution of ant societies. The female mud daubers don’t just lay an egg on the ground and fly away. Instead, they first create a mud cylinder in which to house the egg
The wasps then find a victim to paralyze, bring it to their nest and seal the cylinder shut. When the egg hatches, the wasp larva can attack its helpless roommate.
The ancestors of ants may have started out as similar insects, Dr. Ward and his colleagues suggest, building simple nests and delivering food to their offspring. Instead of delivering a single meal to their young, the proto-ants may have begun delivering many smaller victims to their young after they hatched. Later, their offspring may have begun to help their mothers raise more offspring.
Dr. Ward doesn’t think it’s a coincidence that the new study reveals bees as having evolved from mud-dauber-like ancestors as well. Bees have also evolved complex societies, in which workers fly off to find pollen and nectar to feed their hives. It may be that only a certain kind of wasp could give rise to either kind of highly social insect.
Dr. Boomsma agreed with this idea. “Colony life with altruistic helpers could only evolve after nests and parental care had evolved first,” he said.
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