Nikola Solic / Reuters file

An exhibit at the Neanderthal Museum in Croatia shows a male working near other family members in a cave.

Research into the roots of our species suggests that stronger bonds between fathers and their children played a role in shaping society as we know it — which is a worthwhile subject for reflection on this Father's Day weekend. Some anthropologists even suggest that current social trends could bring us closer to the good old days of hunter-gatherer fathering.

Although most moms wish the man in the house would take more of a role in childraising, things could be worse. Males take on the job of parental care in only 5 percent of mammalian species, according to research cited in "Fatherhood: Evolution and Parental Behavior." In that book, anthropologists Peter Gray of the University of Nevada at Las Vegas and Kermyt Anderson of the University of Oklahoma tick off several facts about the evolution of fatherhood:

• Men tend to spend more time with young children in hunter-gatherer societies than they do in most other societies, particularly ones in which men make a living herding livestock. "Fathers might have sex-specific reasons for spending time with children, for instance, teaching boys to hunt," Gray and Anderson wrote in The Chronicle Review.
• Men tend to invest more in children if they are the biological parents rather than the stepparents. "Spending time on offspring who do not carry any of your genes seems counterintuitive," the anthropologists said.
• Some studies have shown fathers tend to have lower testosterone levels than men without children in the same society. "This could be picking up the behavioral transition men undergo as fathers as they move away from some degree of investment in courting women, seeking new mates, competing with other males, and begin to settle into a more family-oriented outlook, including spending time directly with a young child," Gray told the Boston Globe.
• Paternal care is largely absent among our primate cousins, and the fossil record suggests that the common ancestors of humans and chimpanzees were similarly fatherhood-deficient.

So what changed? Anthropologists say the rise of male-female pair bonding was a key factor. "We see that process getting under way around 2 million years ago but becoming more pronounced around a half a million years ago," Gray and Anderson write. With pair bonding, males had a bigger stake in defending their mates as well as their genetic progeny.

When fathers invested more time in nurturing the family group, the mothers didn't have to expend as much energy — and that opened the way to more frequent births and bigger families. At least that's the hypothesis advanced by Northwestern University's Lee T. Gettler in the journal American Anthropologist. That, in turn, led to a population explosion among the early members of the Homo genus.

How males gathered together
Other research suggests that early humans diverged from chimps in the organization of hunter-gatherer societies. Groups of chimpanzees are generally organized along kinship lines, and there's a high level of aggression between those kin groups. But Arizona State University anthropologist Kim Hill and his colleagues reported in the journal Science that today's human hunter-gatherer groups are more mixed up, genetically speaking.

Such intermixing, coupled with pair bonding, might well explain why intergroup relations go more smoothly among humans than they do among chimps (even though we still have a long way to go). A father who recognized his son in a neighboring group would be less likely to strike out against him, which would open the way for larger tribal networks. (Thanks to Blogging the Bust's James Lavin for making the connection to fatherhood.)

Last month, University of Colorado anthropologist Sandi Copeland and her colleagues published a paper in the journal Nature suggesting that hominid males tended to be stay-at-home types who stuck together as far back as 2 million years ago. The researchers' conclusions, based on an analysis of fossil teeth found in South African caves, lend weight to the idea that cooperative behavior among males was one of the hallmarks of hominid society.

The takeaway message is that fatherhood and male bonding took on a higher profile in the formation of hominid hunter-gatherer societies, even before the rise of Homo sapiens — and that those aspects contributed to the organization of early human society, on a scale that could be larger than that found among other primates to this day.

Modern-day message
So what does that mean for modern-day fathers? You could argue that we're taking on more of the aspects of a hunter-gatherer social setting thanks to some of the trends we're seeing today — more women in the workplace, more stay-at-home dads, blurred roles in childrearing, a departure from industrial work patterns, even increased connectedness through mobile devices.

Gray told the Globe that today's society may be friendlier to fatherhood than the family setting in the 1960s, portrayed so vividly (and unflatteringly) in shows such as "Mad Men." In short, this is not your father's fatherhood.

"One thing presented in a show like 'Mad Men' that was also a common family pattern in the baby-boom era in the U.S. is this sexual division of labor," Gray said. "Women were at home and didn't work. That does not apply well to an evolutionary backdrop. Among hunter-gatherers, women and men are both working, but in ways compatible with having young kids."

Has fatherhood evolved, or are we still cavemen when it comes to parenting? Feel free to weigh in with your comments below ... and have a great Father's Day!

More about the science of fatherhood:

• Meet the mice with two dads
• New dads get baby blues, too
• Role of American dads is changing
• Dads get a hormone boost from child care

You can connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. Also, give a look to "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Darryl de Ruiter

This specimen, known as SK 48, is one of the best examples of Paranthropus robustus from South Africa's Swartkrans Cave.

An analysis of fossil teeth from South Africa suggests that the males in pre-human societies stayed near the caves where they grew up, while the females migrated when it was time to mate.

The researchers behind the analysis say their findings, published in this week's issue of the journal Nature, could shed light on the migratory behaviors that eventually gave rise to human societies.

"This appears to be most similar to a chimpanzee social structure," said lead author Sandi Copeland, an anthropologist at the University of Colorado and Germany's Max Planck Institute for Evolutionary Anthropology. It's also consistent with the way many humans handle the process of moving out and settling down, she told journalists. "It's more often than not in modern societies that the woman is the one that leaves," she noted.

But it's not the norm in the animal world. "In most primates and most mammals, it's usually the males who leave their home community," she said. Thus, the analysis could hint at one of the factors that made us human.

Darryl de Ruiter

The skull of "Mrs. Ples" is the most famous example of Australopithecus africanus from South Africa's Sterkfontein Cave.

The research is the result of a "continuing effort that we have been making over the last five to 10 years," focusing on an area of South Africa that's rich with caves and pre-human fossils, said Oxford University archaeologist Julia Lee-Thorp, another one of the study's co-authors. She and her colleagues have been measuring the radioisotope distribution around the Sterkfontein and Swartkans cave sites, to come up with the chemical fingerprints for samples in that area.

The point behind this exercise is to chart out the migration patterns for species that can no longer tell their own tales. By sampling the isotopes of strontium in tooth enamel, the team could determine which teeth belonged to animals that grew up in the cave region and absorbed strontium in the characteristic isotopic proportions from the foods they ate. That chemical signature remained intact in the tooth enamel throughout adulthood, even if the animals later roamed to a different area.

Sandi Copeland

Faint marks, left behind by a laser ablation procedure, can be seen along the right side of this Australopithecus africanus molar.

The researchers carefully blasted 19 pre-human teeth with pinpoint laser beams, and then analyzed the results. Eight of the teeth came from the Sterkfontein Cave and were traced to a species known as Australopithecus africanus, dating back about 2.2 million years. The other 11 teeth came from the Swartkans Cave, and are attributed to Parathropus robustus, a species that lived about 1.8 million years ago. Australopithecus is thought to be a closer ancestor of modern humans than Paranthropus.

The team expected to see a difference in the local vs. non-local distribution of the two species. They didn't.

"As the numbers rolled off the mass spectrometer after each laser ablation, we were at first disappointed," Lee-Thorp said in an Oxford news release. "But we soon realized that we had found another prize — a difference between the males and females. It was totally unexpected."

The researchers assumed that nine significantly larger teeth in their sample came from males, while the 10 significantly smaller teeth came from females. They chose these teeth for comparison precisely because of the size difference.

Based on the strontium analysis, more than half of the female-sized teeth came from outside the cave region, while 90 percent of the male-sized teeth were traced to the caves. That finding led the researchers to conclude that the males were stay-at-home types, while the females were more likely to roam.

Such a pattern is similar to that seen in chimpanzee societies, where males in a particular locale tend to stick together to defend their turf from interlopers. In order to guard against inbreeding, the younger females are likelier than the males to migrate for mating. Copeland said the situation is different for gorillas. In those societies, the dominant male gorilla rules over a harem that tends to stay put, while younger males usually have to go someplace else to find their own mates.

"This study gets us closer to understanding the social structure of ancient hominds, since we now have a better idea about the dispersal patterns," Copeland said in a University of Colorado news release.

The team's conclusions are based on less than two dozen teeth, divided up into species and sex categories. That raised questions about the small sample size. Texas A&M anthropologist Darryl de Ruiter, a study co-author, acknowledged that the team was "very constrained by the amount of material that we have available for destructive analysis." But he said that adding a few more teeth to the sample size may not have helped, because they had already selected the largest and the smallest teeth available to them.

"Anything else we would have added would have been in the gray area," he told journalists. "It wouldn't have added any value to the discrimination between males and females."

More about human origins:

• Lucy's 'great-grandfather' found
• How big babies shaped society
• Can fingers point to sex habits?
• Interactive: Before and after humans

In addition to Copeland, Lee-Thorp and de Ruiter, co-authors of "Strontium Isotope Evidence for Landscape Use by Early Hominins" include Matt Sponheimer, Daryl Codron, Petrus J. le Roux, Vaughan Grimes and Michael P. Richards.

You can connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. Also, give a look to "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

AAAS / Science file

Researchers measured the length of fossilized fingers from Ardipithecus and other ancestors on humanity's family tree, then compared them with modern-day species in hopes of figuring out how aggressive and promiscuous long-gone species might have been.

The oldest-known species on humanity's family tree was built to be pushy and promiscuous, while another long-ago ancestor known as Lucy was lovey-dovey. Early humans and Neanderthals were more competitive than we are. At least those are the conclusions that researchers reached after sizing up the fingers of extinct and present-day primates.

The study, published in the Proceedings of the Royal Society B, draws upon a famous and controversial indicator of social behavior: the comparative length of the index finger and the ring finger, also known as the 2D:4D ratio.

If the ring finger is longer than the index finger, that's supposed to be correlated with higher prenatal exposure to androgens -- resulting in a higher proclivity for aggressiveness and promiscuity. The finger-length ratio also has been linked to sexual orientation as well as sporting prowess and musical ability.

(Did you just look at your fingers?)

Emma Nelson, an archaeologist from the University of Liverpool, extended the finger-length ratio study to a wide range of species. She and her colleagues measured bones from modern-day species such as gorillas, chimps, orangutans and the white-handed gibbon. They also looked at fossil bones or previously recorded measurements from extinct hominids ranging from Neanderthals (which co-existed with humans until about 30,000 years ago) to Australopithecus afarensis (Lucy's species, going back 3.1 million years) and Ardipithecus ramidus (the oldest human-linked fossil species, going back 4.4 million years).

Nelson argues that comparing the finger-length ratios of extinct and present-day species is a valid technique for making an indirect assessment of our long-gone ancestors' social behavior.

"It is believed that prenatal androgens affect the genes responsible for the development of fingers, toes and the reproductive system," she explained in a news release. "We have recently shown that promiscuous primate species have low index-to-ring finger ratios, while monogamous species have high ratios. We used this information to estimate the social behavior of extinct apes and hominins."

Nelson previewed her findings a year ago at the Society of Vertebrate Paleontology's annual meeting, where she talked about naughty Neanderthals and monogamous australopiths. The newly published paper draws upon additional samples, including the finger lengths for Ardipithecus, or "Ardi." So here are the numbers:

• Modern humans averaged a 0.957 index-to-ring finger ratio, and were considered to be on the line between a "pair-bonded," or monogamous, species and a middle-of-the-road species.
• Chimps, gorillas and orangutans had index-to-ring ratios in the 0.90 to 0.92 range, and were classified as "non-pair-bonded," or promiscuous.
• An early modern human from Israel's Qafzeh Cave, thought to be about 95,000 years old, had an index-to-ring ratio of 0.935. Based on that statistic, the researchers surmised this individual would be more promiscuous than modern humans.
• The finger bones from five Neanderthals yielded a 0.928 ratio, associated with even greater competitiveness and promiscuity. Ardipithecus' bones took it up another notch, to 0.899. Two even older primate ancestors, Hispanopithecus and Pierolapithecus, had ratios of 0.848 and 0.908, which means they would have been tough to live with as well.
• On the other end of the spectrum, the monogamous gibbons had a 1.009 ratio ... and the australopith sample came in with a ratio higher than that of modern-day humans (0.979). The implication, then, is that australopiths were monogamous.

The big question is whether there's anything substantial to this analysis. Nelson acknowledged that the fossil record was sparse, and that more fossils were needed for study, but she insisted that "this method could prove to be an exciting new way of understanding how our social behavior has evolved."

Other researchers have tried to make guesses about the social behavior of extinct hominid species by looking at sexual dimorphism -- that is, the differences between the male and the female of a species. If the males were significantly larger, the assumption is that they were built to have their way with many females in a promiscuous social setting. This has generated a fair amount of debate, particularly when it comes to assumptions about australopiths.

Nelson and her colleagues suggest that the finger-length ratio could serve as an additional tool for making more educated guesses about ancient social behavior.

"Social behaviors are notoriously difficult to identify in the fossil record," one of Nelson's fellow authors, the University of Oxford's Susanne Shultz, said in the news release. "Developing novel approaches, such as finger ratios, can help inform the current debate surrounding the social systems of the earliest human ancestors."

When this research first came to light last year, University of Wisconsin anthropologist John Hawks cautioned against reading too much into fossilized fingers. He said the index-to-ring ratio "may be correlated with mating system in primates, but that doesn't mean it's a good predictor of mating system. ... As fossil hominins go, I wouldn't expect the story to go any further -- there just aren't many hands, so there's never going to be a significantly predictive result."

Be sure to read Hawks' posting from last year, and feel free to weigh in with your comments below ... that is, after you've finished checking out your fingers.

Update for 2 a.m. ET Nov. 3: John Hawks provided some additional thoughts in response to my e-mail inquiry:

"The 2D:4D ratio really is a subject of a lot of research in psychology and developmental biology, and it really does reflect prenatal hormone exposure. However, it is not a good predictor of any social behaviors.

"In addition to the problem of using a poor predictor, this study has another problem that we often face with fossils -- there are very few of them, and it's not obvious which sample of living primates we should be comparing with them.

"The living apes do vary in mating structure, but they also have huge differences in hand anatomy because of locomotion. Those anatomical differences between species do not necessarily have any relationship to the neurological correlates of prenatal hormones -- even though the variation in hormone exposure is an important cause of variation within species.

"For example, Ardipithecus has fingers and hand proportions that are right within the range of other apes. So when we see that they have a 2D:4D ratio right in the range of other apes, the natural hypothesis is that this reflects their overall hand proportions. Australopithecus, Neandertals and modern humans obviously had humanlike hand proportions, and their 2D:4D ratio may simply reflect this.

"If you were going to do this study right, you would look far beyond the apes to take in many kinds of primates with different social systems. Then you could see whether closely related species have 2D:4D ratios that track their mating systems. Without this, we are really looking at a single evolutionary event -- the rise of the hominins -- which may be unique for many reasons besides mating system."

In addition to Nelson and Shultz, the authors of "Digit Ratios Predict Polygyny in Early Apes, Ardipithecus, Neanderthals and Early Modern Humans but Not in Australopithecus"  include Campbell Rolian and Lisa Cashmore.

Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also check out "The Case for Pluto," Alan's book about the controversial dwarf planet and the search for new worlds.