How Much Did Eating Meat Really Shape Human Evolution?

IRA FLATOW: Scientists have long theorized that meat– meat– is what made us human. This is how the story goes. About 2 million years ago, Homo erectus, an early human ancestor, emerged. Homo erectus had a bigger brain, longer legs, and a smaller gut, much more human than ape. And the cause of these big evolutionary changes? Eating more meat. Yes.

Now, a group of researchers has re-analyzed the fossil record and is starting to question the assertion that meat eating was the primary driver of changes during this pivotal point in human evolution. Joining me now to talk more about the significance of these findings is the study’s co-author, Briana Pobiner, paleoanthropologist at the renowned Smithsonian National Museum of Natural History in Washington. Welcome back to Science Friday.

BRIANA POBINER: Thanks for having me. Glad to be here.

IRA FLATOW: Nice to have you. OK, let’s start off. What’s the reasoning behind the “meat makes us human” theory?

BRIANA POBINER: Well, the idea is that we see the evolution of this species you mentioned, Homo erectus, about 2 million years ago. And a little bit before that, maybe around 2 and 1/2 million years ago, the archaeological record shows that stone tools are being made and animals are being butchered, big animals, to eat meat.

And so the conventional wisdom has been that since meat eating occurred a little bit before, kind of around the same time, as the evolution of Homo erectus, that the physical changes and biological changes we see with the evolution of this species are closely tied to this big shift in diet.

IRA FLATOW: It’s not that Homo erectus was the first early human to eat meat. It’s that the fossil record seems to show that a spike in the amount of meat, right, in their diets.

BRIANA POBINER: Exactly. We only see occasional meat eating, really, before Homo erectus. And then once about 2 million years comes around, we all of a sudden see some archaeological sites that seem to have a lot of evidence for meat eating. So the interpretation of this increase in volume, in absolute volume, of butchered bone has always been, well, it’s got to be a reflection of behavior, of a change in diet, of more meat eating.

IRA FLATOW: So what did you discover that led you to believe that this theory may not be as airtight as it originally seemed?

BRIANA POBINER: So it turns out that if you look at basically how many fossils have been dug up, the fossil record, paleontological sampling, that really increases in the same time periods that we see this big spike in evidence for meat eating. So it looks like it’s actually just tracking sampling and that the evidence for meat eating doesn’t increase substantially with the evolution of Homo erectus and stay high. There are some spikes, but it really just tracks that sampling. It seems to be really a kind of a sampling signal instead of a behavioral signal.

IRA FLATOW: Aha, so that’s really interesting. I have to ask you, though, what made you decide to look back into the fossil records? I mean, did you have an inkling that maybe there wasn’t as much evidence as you thought there might be to suggest that eating more meat drove major human evolution?

BRIANA POBINER: So the first author of the study, Andrew Barr, contacted me and said he was assembling a group of people who were interested in basically testing this conventional wisdom. And I came on as the zooarchaeologist, somebody that studies the butchered animal fossil record. I’ve excavated sites with butchered animal bones for a couple of decades, and I was really familiar with the evidence. So I was able to bring that side of the evidence in. And some of the other co-authors looked at the paleontological sampling and kind of wove the whole story together.

IRA FLATOW: And what did you actually think you would find versus what you found?

BRIANA POBINER: Well, I thought that the conventional wisdom would hold up. I thought that we would see this bigger increase than would be expected just from changes in sampling. So I was really surprised by our findings, actually.

IRA FLATOW: And some anthropologists have questioned the conclusions, correct, that you drew based on your analysis of the fossil record. And their argument is that maybe there’s less evidence of early human butchering animals because it was just less common, and it’s not a matter of oversampling those 2 million-year-old records.

BRIANA POBINER: Well, and that still could be. I mean, basically, our conclusions are that currently the evidence doesn’t support this big behavioral shift at 2 million years ago, a huge increase in the volume of meat eating compared to prior time periods. And ironically, it actually makes the meat eating before 2 million years ago look pretty impressive, because there isn’t a big jump at 2 million years ago. There’s kind of this sustained level of meat eating.

IRA FLATOW: OK, so if it wasn’t necessarily an uptick in meat eating that caused this major change, what are some other explanations that might be plausible?

BRIANA POBINER: So there are some alternative hypotheses or explanations. One is that there’s plant foods, maybe underground storage organs like tubers, that that was a major food source that could have led to these changes. Another idea is that it’s cooking and fire, and being able to extract more nutrients from the same foods or eat foods we wouldn’t have otherwise.

And another hypothesis, maybe, is that it’s eating insects as kind of an alternative protein source. The problem is that there isn’t really good visible archaeological evidence for any of these alternative hypotheses back in the archaeological record this far.

IRA FLATOW: There’s a big recent fad in eating, and it’s called paleo diets, where people don’t eat anything other than veggies and meat. They don’t have grains or processed foods or dairy products. And they claim that they’re trying to get closer to what our ancient human ancestors might have eaten. And based on this study and your past research in ancient human diets, what are these diets getting wrong?

BRIANA POBINER: There’s a lot of assumptions in the modern paleo diet movement. And I should first say that none of us on this study had any kind of paleo diet agenda that we were looking to find evidence for or against the idea of eating a lot of meat. But really, the idea of excluding food sources– I can’t imagine any early humans looking at potential food sources and going, well, we’re not adapted to eating that. I don’t think we’re going to eat that today.

IRA FLATOW: It’s not in my diet today.

BRIANA POBINER: Exactly. It’s not on the menu. Early human diets were really about expanding, actually, diet breadth and food sources. And interestingly, there are some ideas that our species, modern humans, were so successful and may have even out-competed some other species because we ate a wide variety of food.

IRA FLATOW: Can’t be too picky. OK, so what’s the next step? What research is needed, do you think, to better understand the role of meat eating in human evolution?

BRIANA POBINER: Well, it would be great to have more fossil samples from before 2 million years ago, particularly if we could find archaeological sites that had lots of layers where bones were preserved. And we could see, if it’s not just a handful of bones from one area on the surface or one layer, are early humans in these earlier time periods going back over and over again to butcher dozens of animals? That would maybe change the story. So as usual, it’s, let’s get out there and find some more evidence.

IRA FLATOW: And you’ll come back and talk about it, won’t you, Briana?

BRIANA POBINER: I’d love to.

IRA FLATOW: Briana, thank you for taking time to be with us today.

BRIANA POBINER: Thanks for having me.

IRA FLATOW: Dr. Briana Pobiner, paleoanthropologist at the Smithsonian National Museum of Natural History in Washington.

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