Ten scratches on two bone fragments distinguish vegetarians from carnivores

It seems like everyone wants to know when we started eating animals, and why we started eating animals, and how that affected our evolution downstream. (Or, if you’d rather, upstream.)
Even if they’re not intrinsically interested in human evolution, most people (including Ozzy over there) still want to learn more about some of our species’ favorite pastimes: Eating and killing things.

If you're ever visiting the gorillas at the zoo, it's pretty obvious what humans are interested in. Of the juvenile chewing on lettuce and carrots, a mother says to her toddler, “Look sweetie, he’s eating his vegetables like a good little boy,” and a teen-aged girl says to her brother, “See, they don’t eat meat and since we share, like, most the genome with them we shouldn’t eat meat either.” Of the silverback male sitting quietly in the corner, a man observes, “Dude’s large and in-charge. I wouldn’t want to [bleep] with that. He’d kill the [bleep] outta me.”

Most of the profound pronouncements you’ll hear at the zoo’s mirror for humanity are centered around food and violence.

Likewise, many, if not most, of the big-picture explanations for human evolution—for why humans are the way we are, for why humans are unlike other apes—revolve around eating and killing animals. Just to name a few…We walk upright to carry and throw things like tools, which are presumably used for hunting prey and processing carcasses (Darwin).We walk upright to move about as terrestrial foragers (which at some point begins to include scavenging and hunting) on the open savannas (The Savanna Hypothesis). Humanity was born in a hunting past (Washburn and Lancaster). Big brains were able to evolve because of the supplemental nutrition and calories obtained through scavenging and hunting (just about everyone). Scavenging, hunting, and stone-tool manufacture require a larger, more cognitively complex brain (just about everyone).

So it makes sense that paleoanthropologists are desperately searching for evidence of our ancestors eating animals. Especially very early evidence. And, in order to find it, you’ve got to not only collect every scrap of bone that you see while you survey or that you dig up while you excavate, but you’ve got to scrutinize each one of those scraps, preferably under a microscope.

Otherwise, you might miss something like the ten scratches on two bone fragments (one rib and one femur) that were just recently reported in Nature.

These artifacts from the site of Dikika in the Afar Region of Ethiopia are the earliest evidence of hominin (human ancestor) meat-eating. What's more, they're the earliest evidence by nearly one million years! Normally finds in paleoanthropology beat-out others by a mere fraction of this quantum leap.

Cut-marked bone at 3.4 million years ago takes the title of First Meat-Eater away from the genus Homo (Homo habilis, Homo erectus,.... , Homo sapiens) and gives it to the only hominin species known at the time from that region of Ethiopia: Australopithecus afarensis, known for being Lucy’s species.

If you're not quaking in your seat right now, well, then you have got some serious nerding-up to do. This is kind of HUGE: Lucy’s relatives, and maybe Lucy herself, not only used stone tools but used them on big mammal carcasses!?

Until this discovery, all we had were stone tools at 2.6 mya and cut-marked bones at 2.5 mya and everyone was content in thinking that the major behavioral shift associated with the human genus, Homo, was obtaining ever-increasing amounts of animal fats and proteins into our diets. That's the best way to make sense of the trend for encephalization which is so much more pronounced in Homo than in previous hominins (where brain sizes were basically chimp-like for the four million years that our ancestors were undergoing selection for increasingly efficient bipedal locomotion).

And see... that has been a little bit of problem: Why would natural selection continue to hone our bodies for bipedal locomotion if not to use them for hunting? With this new evidence for meat-eating earlier in our past, bipedalism makes a lot more sense. At least from the traditional standpoint.

But you’re probably thinking… ten scratches on just two bones? Well, it’s come down to less than this before. When the evidence is good, you certainly don’t need more than a meal’s-worth to show that an animal was butchered with stone tools.

Even if those stone tools were not shaped or chipped or flaked first, even if they were just rocks that somebody grabbed from a dried-up riverbed, nobody but us and our ancestors uses/used stone tools on animals. (Okay, otters use rocks to bash open sea urchins and other snacks but not bones of East African bovids.) Chimpanzees use rocks to crack open nuts, but when they occasionally hunt prey they either use sticks as spears or they simply use their strong bare hands, like baboons are known to do as well. So, bones with scratch marks that were made by stone tools can’t be interpreted any other way than to have been made by human ancestors, or at least relatives of our ancestors (collectively termed hominins or hominids, depending on preference).

Except, EXCEPT!, there are some who would still disagree and they may make the following arguments:

1. Carnivores, like lions and hyenas, can scratch bones of prey with their teeth and if those bones aren’t crushed to smithereens as part of the meal, then they can be discovered millions of years later and trick paleontologists. Okay, but that old chestnut’s been cracked wide open. Microscopic details that distinguish tooth scratches from tool scratches have been established and these bones weren’t scratched by teeth.

2. At any time after the animal’s death, its bones could have been trampled and that could explain the scratch marks. Okay, but again, under a microscope and x-rays, those marks differ from the marks left by tools and, again, these bones weren’t scratched by feet.

So what does this mean for our reconstruction of human evolution?

Well, first of all, like we discussed after the Malapa hominins were announced, the genus Homo is probably going to have a makeover here soon. As of now, the earliest members of the genus Homo are no older than 2.4 million years ago. And now that there are three sites (Bouri, Ethiopia at 2.5 mya, Gona, Ethiopia at 2.6 mya, and now Dikika, Ethiopia at 3.4 mya) with stone tools or cut-marked bones that precede the appearance of Homo fossils, either the behavioral or the anatomical criteria must change or both.

We must now accept the idea that eating meat had a much earlier involvement in our evolutionary history than we thought.

It also means that australopiths, with their ape-sized brains, were performing activities that we have always considered pretty difficult. And this cognitive complexity came not only without the aide of human brains but without the aide of human trainers!

And finally, if you ever have a chance to have Lucy over for dinner, now you know what she likes: BBQ ribs and rump roast. Good thing, because vegetarian dinner-party guests can be so persnickety and, oh boy, does that beat the heck out of underground storage organs!


References
Shannon P. McPherron et al. 2010. Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature 466: 857-860

Repeat after us: Correlation is not causation, correlation is not causation, correlation is not....

Confounding is probably the single most important explanation for irreproducible and even nonsensical results in epidemiology--and probably in genetics and evolutionary reconstructions as well. In the mid 1900's, for example, before everyone had telephones, researchers found that having a phone was a risk factor for breast cancer. How could that be? As it turned out, having a phone was associated with middle or upper class status, and money was associated with a diet that included increasingly more fat--or with increased age at first birth, or fewer children, and so on--risk factors for breast cancer subsequently identified by many studies that noted increased risk as socioeconomic status rose. Confounding is notoriously difficult to control, largely because many associations can't be anticipated in advance of the design of a study. (Whether the same-sounding argument applies to the idea that use of cell phones 'causes' brain cancer is not known.)

The BBC is reporting that meat eating causes early menarche. Or rather, eating a lot of meat. This according to a paper in Public Health Nutrition (though this is only the latest of the papers reporting this correlation). The age at menarche--first menstrual period--dropped throughout the 20th century and many people have wondered why. It had been thought that this was due to increased nutrition in general, but arguing against this idea is the observation that, as obesity rates increased, age at menarche didn't further decrease. That is, it's apparently not a simple matter of body size or nutritional intake.

The question of what has caused early periods prompted a group of researchers in Britain to look prospectively at a cohort of 3000 girls, including nutritional intake at age 3, 7 and 10. They identified a group of girls at birth in 1991 or 1992, and followed them up for about 13 years both by questionnaire and clinically. Early menarche was considered 12 years 8 months or younger, experienced by about half of the sample.

Girls with 'high' meat intake were eating 8 or more portions of meat per week at age 3 and 12 or more portions at age 7. Since early menarche has been associated with increased risk of breast cancer (the odds ratio is 1.5 - 2 times higher for women who started having periods at or before age 12 vs. women who started at 15 or older), the authors of the paper note that early menarche should be of concern. (But remember that 1.5 - 2 times a fairly low risk is not that high, and that, anyway, the comparison group, women who reached menarche at 15 or older has been a very small fraction of women for decades, at least in the developed world, and they perhaps were late for reasons that also protect against breast cancer, or at least may have different hormonal profiles. So whether this increased odds ratio is meaningful is up to you to decide.)In this large group of contemporary girls we have found a number of associations between dietary intakes throughout childhood and the occurrence of menarche. We have confirmed previous findings of higher energy intakes among girls reaching menarche earlier, reflecting their larger body size. We have also found evidence that intakes of meat and total and animal protein, and also possibly PUFA [polyunsaturated fatty acids] in early to mid-childhood may increase the chances of menarche by 12 years 8 months. However, we found no evidence that the chances of reaching menarche increased with higher total fat intakes, or reduced with higher intakes of fruit, vegetables or NSP. Unexpectedly, higher vegetable intakes at 3 years were associated with an increased chance of reaching menarche, although this may have reflected the positive association between meat and vegetable intakes at 3 years.

But could the meat/menarche association be due to confounders, variables that are associated with meat consumption and are the true explanation for the correlation? Things like ethnicity, socioeconomic status, mother's behavior during pregnancy, and so on, which would influence diet? The researchers controlled for some of these, but ethnicity was classified crudely as white/non-white, for example, and in the UK, as elsewhere, non-white can cover a lot of different diets, so that something else that rides along with meat could be the explanation instead. Kind of cooking oil, for example--we have absolutely no evidence that this is the case, but the point is that it's possible. Not to mention that high meat consumption is often associated with increased consumption of processed foods in general, which was not controlled for.  That is, high meat consumption could indicate a different diet, with some unidentified causative variable, compared with the diet of kids who eat less meat.

Another possible confounding variable that has long been discussed in the literature--though as far as we can tell, the association is still only suggestive--is exogenous hormone levels in meat (a brief time out for a bow to the web, where you can find just about anything: here, for example, is a link to the "Museum of Menstruation and Women's Health", which we stumbled upon in searching for papers on meat hormones and age at menarche). Animals, of course, are given a panorama of hormones to increase the speed at which they grow. Whether these hormones are found in excess in meat, or are active in our bodies when we eat that meat isn't clear, at least to us. But it's certainly a possible alternative explanation for the association between meat consumption and age at menarche.

So, should we stop feeding meat to little girls? Early age at menarche, whatever its cause, may be a risk factor for breast cancer (any increased risk is not to be taken lightly, though see above), but it has also been found to protect against osteoporosis--both presumed to be due to the increased length of exposure to estrogen. And, of course age at menarche is by no means the only factor associated with risk of breast cancer or osteoporosis. And the protein and other ingredients in meat are likely good for other aspects of growth and health.

Of course epidemiologists (and journal editors) have to eat, and to do that they have to publish lots of studies!  Does this study tell us anything of significance? You decide.