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I am a politically-progressive, ethically-herbivorous anthropoid pursuing a paleontology education in the Los Angeles Basin. I am largely nocturnal, have rarely been photographed, and cannot thrive in captivity.

07 May 2011

Back Atcha, PermaVegan! -- Fallback Food Fight, Round 2

Image courtesy of Kenyathropus.com
I should thank the PermaVegan for engaging me in this debate, for it has goaded me into performing one of the most essential elements of scientific thinking: arguing against one's own bias. In contemplating and researching a response to his latest entry, I realized that I, like many vegans, was clinging to the hope that our hominin ancestors were herbivorous. Consciously, I knew this wasn't exactly true, but emotionally, I wanted it to be true. And thus, what I'm about to do feels like a betrayal, even though it isn't. Veganism, as I've said, is a modern ethical stance, formulated in response to modern challenges. What our ancestors ate isn't morally or ethically relevant.

Nonetheless, the PermaVegan asks a good question, in four parts, that deserves a robust answer:
My question, though simple -  "How do you resolve the abrupt evolutionary reversal that appears to be implied in a proposed hominin preference for meat?" - is best introduced in four parts: 

  1. As you read the scientific literature, what is the earliest date (in millions of years ago, or Mya) for which you think it is reasonable to argue that our line of descent was frugivorous/folivorous?
  2. By what date, approximately, do you believe our line of descent can be said to have demonstrated a dietary preference for meat over fruits and vegetables?
  3. How do you argue that a new preference for the consumption of meat over fruits and vegetables arose prior to a generalized or plant-foraging adaptation that only secondarily made meat acquisition a marginally better energy deal in times of fruit and vegetable scarcity?    
  4. As a result of what major environmental and/or genetic modification do you believe the consumption of meat became a more optimal foraging strategy (in net energy terms) than the acquisition of previously preferred plant-based foods?       
This question -- and all four of its sub-questions -- seem to me premised on a single, probably flawed, assumption: that there was ever a time that hominins didn't eat other animals. I am skeptical that any such time ever existed. As far as we can tell, hominins (as opposed to hominids; see the addendum, below) have always been at least somewhat behaviorally carnivorous.

Before getting into the meat of the matter, as it were, I should offer a few caveats, all of which you should keep in mind whenever you read any article on the subject of diet and human evolution:
  1. The fossil record of hominin evolution is spotty, relying heavily on incomplete skeletal remains and teeth. Thus, conclusions about diet drawn from it are tentative. 
  2. The fossil record is biased in favor of meat-eating evidence. Bones and teeth fossilize more readily than plant material or soft body parts. This, again, is a reason to draw only tentative conclusions about the relative importance of meat in hominin diets; aside from isotopic data (which has its own issues), evidence about the prominence of plant foods in hominin diets simply doesn't preserve as well. Thus, we run the risk of over-stating the case for meat.
  3. The evolutionary relationships between hominin taxa are murky, and still controversial. Anthropologists still argue over which lineages are ancestral to Homo. So, again, conclusions must be tentative.
Bearing that in mind, however, the fossil record does give us good indications of early hominin diets... but the catch is deciding precisely which hominin taxa are directly relevant to the case. It's always been assumed (reasonably) that at least some of the australopithecines were direct ancestors of genus Homo. As a result, the debate over just how carnivorous our ancestors were has tended to focus on the question of which australopith was most likely to be our ancestor. Those who preferred the interpretation that A. afarensis was a direct ancestor of all subsequent hominins gave points to the vegan camp (whether they knew it or not), while those who preferred the interpretation that A. africanus wasn't descended directly from A. afarensis but was ancestral to Homo gave points to the carnist camp (whether they knew it or not).

But, a few recent fossil finds have cast doubt on whether the australopiths were actually our ancestors at all; turns out, they might have just been a sister taxa to the Homo line, and thus an evolutionary dead-end with no direct relevance to us.Which, if true, means that the hominins actually related to us were (slightly) more carnivorous than their australopith cousins.

Consider Kenyanthropus platyops, discovered in 1999 by Justus Erus, working on Maeve Leakey's team (yes, of those Leakeys!) at Lake Turkana, Kenya. This enigmatic hominin discovery sparked a debate that rages to this day. Leakey's team suggested then, and still do today, that Kenyathropus deserved its own genus, and may have been an ancestor of Homo rudolfensis. This suggestion would take Homo out of the australopith line of descent altogether, and turn australopiths into an evolutionary dead-end; a radical move! Understandably, other paleoanthropologists dispute their findings, and the issue has not been settled.

Kenyanthropus and its descendants had teeth adapted to a diet of soft, rich foods like tubers, fruits and meat. And if it really is its own genus separate from australopithecines, as the Leakey team suggests, then its dental adaptations would have been inherited from as-yet-undiscovered, non-australopithecine hominin ancestors going all the way back to the chimp-human divergence. And that means, in turn, that the omnivorous capability of our Homo lineage has been with us for 7 million years.

While we're on the topic of omnivory, now would be a good time to point out another concept that needs clarification: the presumed conflict between a frugivorous and an ominvorous morphology. The PermaVegan -- and many other fellow vegans, as well as most carnist armchair anthropologists -- talk about them as if they are different things, but they're not. At least, not exactly.

Among mammals, herbivory and carnivory are both highly specialized adaptations, viewable in the dental and (among extant species) gut morphologies of the relevant species. Frugivory has always been considered intermediate between the two; as Hladik, et. al., noted in their critique of the expensive-tissue hypothesis:
The “faunivore” trend, as well as the “folivore” trend, are morphological specialization – corresponding to different allometric relationships – that are not likely to allow a large plasticity, as for any specialized character. A specialized carnivorous adaptation in humans that would correspond to a minimized gut size is obviously not supported by our data (fig. 1). Large variations presently observed in human diets (Hladik and Simmen, 1996) are probably allowed by our gut morphology as an unspecialized type of “frugivore”, a flexibility allowing Pygmies, Inuit, and several other populations, present and past, to feed extensively on animal matter, for whom most of the energy is mostly derived from fat (Speth, 1987).
In other words, modern human guts are adapted to a diet of soft, energy-dense foods, a condition they inherited from "frugivorous" ancestors but that accidentally also allows them to be better at digesting meat than other primates.  To put it succinctly, H. sapiens are functionally omnivorous because of their frugivory, not in spite of it (a point that threatens to undo the whole debate before it even starts)!

A final point to consider before I answer the PermaVegan's question directly is one I've raised before: the behavioral distinction between "herbivore," "omnivore" and "carnivore" isn't always clear-cut, and isn't always a reflection of skeletal or gut specializations. Witness, as evidence, the strange case of the meat-eating deer; or the lovable panda, a herbivorous carnivore.

So, in answer to the PermaVegan's core question --  "How do you resolve the abrupt evolutionary reversal that appears to be implied in a proposed hominin preference for meat?" -- I'd contend there really isn't any "abrupt reversal" that needs resolving. Hominins have always eaten other animals, to some degree or another. Modern chimps (who are hominids, not hominins, but still closely enough related to us that we can make reasonable inferences) are known to prefer meat whenever it's available to them or whenever they can catch prey; thus, we can reasonably assume that this preference is ancestral to Homo, as well. The burden of proof, I'd say, rests with those who contend that meat-eating is somehow anomalous among hominins... especially if the K. platyops evidence turns out in favor of the Leaky team's interpretation.

Of course, an ancestral behavior of meat-eating does not necessarily imply that meat made up the bulk of an extinct hominin species' daily diet; a great deal remains open to interpretation, and bias can always creep in. Which brings us to the PermaVegan's sub-questions:

1) As you read the scientific literature, what is the earliest date (in millions of years ago, or Mya) for which you think it is reasonable to argue that our line of descent was frugivorous/folivorous?
I'd say our line of descent has been frugivorous (not folivorous! -- they are different adaptions, and using them in the slash style, implying they're the same thing, is inaccurate) since about 55 Ma, during the Eocene radiation of the adapiformes. Many of these animals' descendants developed folivorous capabilities, it's true, but that's when the first reasonably-identifiable frugivorous adaptations (as distinct from their insectivorous ancestry) are notable.

2)By what date, approximately, do you believe our line of descent can be said to have demonstrated a dietary preference for meat over fruits and vegetables? 
Subject to the caveats I offered earlier, I'd say around the same time as the PermaVegan and professional researchers do -- ca. 2.5 to 1.5 Ma. That's when the fossil record gives us robust, somewhat direct evidence that hominins were eating other animals. But again, it's likely this behavior has been with us right from the start.

3) How do you argue that a new preference for the consumption of meat over fruits and vegetables arose prior to a generalized or plant-foraging adaptation that only secondarily made meat acquisition a marginally better energy deal in times of fruit and vegetable scarcity? 
Again, I'd argue that hominins have always had some preference for meat, and there's nothing "new" about it. This can be inferred from observations of chimpanzee hunting in the wild, and the fossil record of our dental development.  My working mental model is that early hominins relied on fruit and tuber foraging most of the time, but snatched the opportunity for meat whenever they could... in a strategy similar to that of modern chimps.

4) As a result of what major environmental and/or genetic modification do you believe the consumption of meat became a more optimal foraging strategy (in net energy terms) than the acquisition of previously preferred plant-based foods? 
The earliest robust evidence for hominin meat-eating appears towards the end of the Pliocene global cooling, which saw the decline of rain forests and the spread of arid grasslands and savannahs; and extends into the cold Pleistocene. The first hominin dental adaptations to a rich, soft-food diet appear a bit earlier than that, in the mid-Pliocene with K. platyops. I'd say that's when the shift began to happen, and wouldn't want to get more specific than that, because I don't think such a shift would have been "abrupt," as the PermaVegan described it. As noted, it's my evaluation that hominins have always had some preference for meat.

I realize this may not be what a lot of vegans want to hear. It's not what I want to hear, either. In my heart, I wish we had descended from purely herbivorous ancestors; if nothing else, it would make the case for veganism a bit easier. But I cannot ignore the evidence of the fossil record; that would be intellectually dishonest. The case for veganism -- or at least, my case for veganism -- isn't built on the details of human evolution. It doesn't have to be. The systems of animal and environmental exploitation we face today are unprecedented, and veganism is a form of conscientious objection from those systems. That it didn't exist in coherent form prior to the modern age should not trouble vegans any more than the absence of ancient Roman skyscrapers troubles architects.

ADDENDUM: Some readers may be confused by use of the words "hominid" and "hominin." The difference is easy to remember, though. Hominin refers to the ancestral line of Homo and all of its sister taxa except the great apes. Hominid includes Homo, its hominin relatives, and the great apes (and their ancestors). So, chimps and humans are both hominids, but only humans are hominins.

1 comment:

  1. Okay, here are my follow ups.

    This is my main response and these are three "side discussions" (1), (2), and (3).

    ReplyDelete