Meat eating led to faster hominin reproduction

Increased meat consumption was an important part of human evolution. In 7 million years we went from meat making up ~3% of our diet to at least ~20% nowadays. Not counting vegetarians of course. Such an increase provided us with a range of benefits. Better nutrition allowed for increased brain size; we could expand into new areas where plant resources are scarce and it meant we were spreading our risk, not relying too much on one type of resource. And as if all that wasn’t enough, new research purports to have found yet another advantage to eating meat: time until weaning.

The how much of a hunter-gatherer‘s diet consists of meat and fish compared to their latitude. Note how groups living the furthest north (where plant resources are rare) eat nearly 100% meat/fish.

All mammals lactate and provide their young with milk for a certain amount of time. This gives them a chance to grow and develop, maximising their chances for survival. However, whilst such an investment increases the chances for offspring to survive it also increases the length of time until the parent can reproduce again. If she’s continuing to spend resources on her newborn, she can’t invest them in carrying another pregnancy to term.

Thus there is a fundamental trade off between ensuring one child survives at the expense of not having another for a while longer. As such, minimising the length of time spent lactating would be a clear advantage, but only if existing children were still given enough resources to fully develop. This raises two interesting questions evolution wants to solve: what influences the length of lactation and how can it be reduced?

Given this is a post about meat you can probably see where this is going

To try and answer these questions researchers (probably not employed by evolution) looked at 67 animal species – including humans – and compared when weaning was with a range of other factors. This research revealed that brain size of a species and time until weaning were strongly correlated, with larger brained organisms living off their mother’s milk for longer. This correlation remained even after the body size of an animal was accounted for.

The implication of this is that brain size is the crucial factor determining how long an infant has to live off their mother’s milk. Since hominins’ brains have increased over the past 7 million years, this would mean that the length of time until we weaned our young would also increase. In such a situation anything that would reduce the time until weaning would be highly advantageous, increasing the speed of our reproductive cycle.

The relationship between body mass and brain mass and time until weaning. Note how brain mass offers the best correlation (higher r2 number). Black circle = humans.

So the researchers set about finding out what could provide such an advantage. As you’ve probably guessed by now, the answer is meat consumption. They found that carnivores weaned earlier than non-carnivores, with there being little difference between omnivores and herbivores.

Now, here you might spy a slight problem. Humans are omnivores, with non-meat substances still making up a considerable portion of our diet. So, do we eat enough meat to gain the advantages of carnivory? To test this, they used all the data they had gathered to predict the time until weaning for an organism with a human-sized brain who was a non-carnivore. They did the same thing for a hypothetical organism that was a carnivore and checked to see which one closest matched non-hypothetical humans.

The result was that we almost perfectly matched the hypothetical carnivore with a human sized brain. In other words, humans eat enough meat to reduce the time spent breast-feeding their young. My one criticism of this point is that these hypotheticals were calculated when people already knew what the results should be, meaning there is a potential for bias here.

How long one would expect the two hypothetical organisms with human-sized brains to spend breast feeding. The results for human populations are the white dots.

Curiously, the prediction was made off data from regular carnivores that don’t prepare their food like humans do. This would suggest that it is just eating meat which gives us this advantage, with food preparation having little impact. This would mean that early hominins could’ve benefited from eating meat without having to develop fire, utensils etc. An Australopithecine merely scavenging a carcass she finds would gain the advantages of meat eating without having to process it. As such, meat eating could well have been important to even early hominins, allowing our species to compensate for the longer reproductive period associated with larger brains.

Of course, there is still the issue of whether the benefit from meat eating is an adaptation. If a pregnant herbivore was suddenly able to eat a lot of meat, would their time until weaning decrease or must they evolve a response to increased meat consumption first? There is also the question of how much meat is needed to feel the effects. Whilst modern humans enjoy faster reproduction, might an ancestor who consumes less meat still benefit? What is the meat threshold for carnivory to have in an impact? However, those are just interesting follow-ups. Regardless of the results, it is clear that meat eating played a crucial role in enabling our ancestors to reproduce faster and become the successful species we are today.

Cordain L, Miller JB, Eaton SB, Mann N, Holt SH, & Speth JD (2000). Plant-animal subsistence ratios and macronutrient energy estimations in worldwide hunter-gatherer diets. The American journal of clinical nutrition, 71 (3), 682-92 PMID: 10702160

Psouni E, Janke A, & Garwicz M (2012). Impact of carnivory on human development and evolution revealed by a new unifying model of weaning in mammals. PloS one, 7 (4) PMID: 22536316

————
Only when doing a bit of further research into this article did I discover it was the source behind all those “meat eating allowed man to conquer globe” stories a few weeks ago. Sorry I’m late talking about it.