Men and women may be considered equal under law, but there are, of course, differences. Among them is size.

Men, on average, are about 15 percent to 20 percent larger than women. Yet compared with other mammals, that margin is slim. New research suggests it was similarities among the males and females of our early ancestors — not differences — that helped early humans evolve to become the dominant species that we are.

Less difference in size suggests early humans were mostly monogamous, explains Owen Lovejoy, an anthropologist at Kent State University in Ohio and an author of a recently published study in Proceedings of the National Academy of Sciences.

And monogamy, he argues, was early humans' key to success.

Monogamous Ancestors

"What monogamy does is eliminates male to male competition for mates," he said. "It allows for more cooperation and that allows you to take better care of your young."

How do similar body sizes suggest monogamy? In the field of evolution, exaggerated size differences between the sexes mean that males required their huge physiques to compete with one another for mates. Less pronounced size differences, meanwhile, mean males were spending less time fighting and more time taking care of their mates and their young.

The concept of one-female kind of guys among early humans is a new one in the scientific community.

More than 30 years of research had instead painted a picture similar to gorilla and orangutan social systems, in which hefty males guard large harems of females. These earlier studies had compared fossils from a range of sites and concluded that males of Australopithecus afarensis — the group of early hominids that lived about 3 million to 3.6 million years ago — were much bulkier than their female counterparts.

But Clark Spencer Larsen, an anthropologist at Ohio State University in Columbus, says there are problems with these previous conclusions. Body size among any species can vary greatly over thousands of years, so comparing male and female specimens from different periods can be misleading. A similar problem arises when comparing specimens from very different geographical regions, where general sizes may vary by location.

Lovejoy's work avoided these issues by testing a series of individuals from one site who all appeared to have died at the same time. He concluded the sexes were more similar than scientists have thought.

"It's a convincing case," Larsen said of the work.

Using ‘Lucy’ as a Measure

To test for body size among the early humans, Lovejoy and Kent State graduate student Philip Reno focused on specimens from a unique fossil find known as the "First Family Site" in Ethiopia. Researchers believe a group of individuals all perished here in a flash flood or other kind of catastrophic event 3.2 million years ago.

Reno and Lovejoy analyzed the bones and teeth of nine adults found during the 1970s and used the famous fossil known as "Lucy," the most complete fossil among the group, as a reference to estimate body size. They compared all of Lucy's bones to her thigh bone (a good measure of size), came up with a ratio and used that ratio to estimate the size of other individuals in the group. They tested the technique on chimp, gorilla and modern human skeletons and found it was an accurate measure of male and female body sizes.

Based on this analysis, the early humans in Lucy's group showed even less difference in size between males and females than contemporary people.

While some scientists remain critical of Lovejoy's methods, the controversial finding has challenged long-standing theories that monogamy and high levels of cooperation were behaviors that early humans took some time to develop.

"In looking for what it means to be human, cooperation emerges as an important trait," said Larsen. "Now we have new insight in the fossil record that even ancient human ancestors had this level of cooperation."

Men: Poor Polygamists

Before starting the fossil analysis, Lovejoy has long argued that monogamy set human ancestors apart very early in evolutionary timelines. As other evidence, he points to what he describes as "hidden" sexual features among modern human females.

Unlike chimps, gorillas, orangutans and other species, human females have hidden ovulation, meaning their bodies reveal no clear changes to signal they have entered this crucial reproductive stage. He thinks this trait stems from human ancestors' monogamous behavior.

"If early human females were reproductive crypts," said Lovejoy, "that would have made them less appealing to other males and monogamous males would have been less vulnerable to cuckoldry."

Having a faithful mate was important, Lovejoy said, since early males were helping feed their mates and their children — so a male would want to ensure the young were, in fact, his. (Other studies have argued that females were also important foragers, and may have even been the primary providers.)

Males, meanwhile, have comparatively smaller testes and lower sperm counts than chimpanzees and gorillas, Lovejoy says, and that suggests men were not designed to be successful polygamists. "Their sperm count is not high enough, and if you're copulating with a female whose ovulation is hidden, there's a high risk of no reward," he said.

Why Men Are Still Bigger

Even if monogamy among our early ancestors minimized size differences between contemporary men and women, some difference remains. Why? There are many theories, but among them is that while females may have preferred mates who were less inclined to busy themselves fighting other males, a slightly larger male could have been a more productive forager.

Meanwhile, males may have selected slightly smaller females to make sure there was enough food to go around.

"If he was offering food," said Lovejoy, "he would have wanted a female that didn't compete with their infant to eat it."