CARTA: Origins of Genus Homo – Leslie Aiello: Evolution of Human Life History Patterns

– [Announcer] This UCSD TV program is presented by University of California Television. Like what you learn? Visit our website, or follow us on Facebook and Twitter to keep up with the latest programs ♪ [music] ♪ – [Narrator] We are the paradoxical ape Bipedal, naked large-brained. Long the master of fire, tools and language. But still trying to understand ourselves Aware that death is inevitable, yet filled with optimism. We grow up slowly. We hand down knowledge. We empathize and deceive We shape the future from our shared understanding of the past. CARTA brings together experts from diverse disciplines to exchange insights on who we are and how we got here. An exploration made possible by the generosity of humans like you ♪ [music] ♪ – [Leslie] So, one thing that I sort of noticed this afternoon is the number of times some of my earlier workers have been criticized and that’s good, that’s normal It shows how the field is growing and developing and building. And I’ve been asked to talk about the evolution of human life history patterns. And this is also a reasonably new approach to study in human evolution and we can understand a lot about the evolution of the genus Homo by looking at life history and by looking at modern people. So basically, life history is more than just looking at the evolution of the physical body form. What it is is looking at how we got to this body form Life history is your tempo and mode of growth and development. And if you look across the primates, you’ll find a variety of different patterns. And if we delve a little bit into life history, as I said, we can learn a tremendous amount about ourselves. And it starts out in childhood You have a difference in the growth and development pattern that’s very obvious For example, chimpanzees all mature by the time they’re about 12 years old. For humans it takes up until your late teens, 17, 18 years old. And our colleague, Barry Bogan, has been very clear about the tempo differences that you have if you compare a chimpanzee with a human. The chimpanzee graph there shows that you have a rapid decline in the velocity of growth. A relatively small plateau period, that’s the juvenile period. And then another relatively rapid decline in how fast you’re growing. Humans are very different in the corner. We have that same decline in the velocity of growth but we have a much longer plateau period where our children are not growing very rapidly And this is followed by the adolescent growth spurt and then it falls off again This pattern is very unique and it’s very important to our evolution. The last graph is from the work of Kristin Hawkes who’s here in the audience. It shows you not only do you have the difference in the pattern of the growth but we also have a very much longer longevity or life span Now what I find fascinating particularly about the early childhood growth is that it has a lot to do with the brain. We know that humans have brain sizes about three times the size of a chimpanzee and that brain grows and develops at different paces in the two species. Now in humans, as a child is growing, a very young child with a huge head uses about 60% of the energy budget just to support that brain size. And as we grow and our body begins

to grow, the brain growth slows down and the energy balance tips. One of our colleagues, Chris Kozawa, has come up with I think is a brilliant look at the relationship between this brain growth and body growth. In these graphs, the red line, one for males, the second graph for females. The red shows the energy that the brain takes as you move from birth up through five years of age that is one of the most energy expensive periods of brain growth in childhood because of all the mileazation that’s going on and then it drops off. The blue line is the velocity of your somatic growth, your body growth So basically there’s almost a perfect play off between brain growth and somatic growth. And this goes terribly long ways to explaining why we have this extended period of childhood that basically correlates and is a necessary correlate in energetic terms with the growth and development of the large human brain cells. Now we all may think this is fine and good but it’s actually not. So some of our other colleagues from Switzerland, Karin Isler and Carel van Schaik have developed something they called the gray ceiling. And what the importance of this is you can just take so long in your growth and development or you aren’t going to be able to replace your population size. Now think in terms of dependent childhood and if a child is dependent on a mother for five years, six years, seven years, she’s going to come back into fertility. You’re going to extend that inter birth interval. You aren’t going to be able to have enough surviving children to replace your population from one generation to another generation. Okay, this is what they call the gray ceiling And if you look at the graph here, there is a lot of primates. The red dots are the apes with the increasing brain size. And what this shows is that with some of our apes like the orangutans, the gorillas and all, what they’re arguing is they’re hitting the gray ceiling and the only way to avoid that gray ceiling is to somehow increase your reproductive output. Now if we go back to that same little chart I chose earlier, at the bottom is the period of infancy. And in chimpanzees, infants are dependent on their mother, they’re still nursing. They are four to four and a half years old before they’re weaned. In humans, it’s of course very much shorter and this particular graph shows about two years, it varies. But what the take home message of this is is you can have twice as many kids if you shorten that period of infancy. And if you then shorten your interval between birth. Now again you may think this is fine, this is a good trick, we’ve all developed it, but it has tremendous problems. And I’ve got this picture hanging in my office and whenever I think I’m having a hard day I think of her. Because what you have once you start to double up this is you have a woman who may be pregnant. The infant that she’s probably in the last stage of nursing and a dependent kid. And she has to get the food to support her own large body weight, her own large brain size, and to provide all of the calories for the kids she’s nursing. She has to provide the food for the young infant. Now this is very different from what we see in the chimpanzees or other apes where mothers focus on one infant. And once the infant has stopped nursing they will gather their own food. There’s very little energetic involvement with the mother. Now this is a huge paradox and this is where the evolution of sharing and cooperation comes in. And of course this is where we have the grandmother hypothesis and the argument that a lot of the extra resources for the grandkids come from the grandmother by provisioning the mother and the kids. Now there’s a number of other arguments also that involved greater cooperation among all individuals in the society, males as well as sibling care and

sibling cooperation. The take home message is that once you begin to develop a large brain size, you have growth and development implications. And these growth and development implications have implications for the evolution of what we would recognize as a human type of cooperative social organization. Okay, the big question is, can we tell when this all happened in human evolution? And we’d like to think, oh, this is a characteristic of the root of the genus Homo, that’s a hypothesis. Now if we go back to the evolutionary tree, we have one way into this in terms of cooperation. And if we go back to the gray ceiling and I know Burden isn’t going to like my brain size against a million year graph. But if we put the gray ceiling onto this and again what Isler and van Schaik argue is that gray ceiling is about 700 milliliters. Now of course there is, you know, big ranges of variation around this, but what’s interesting about this is this just about divides our Homo erectus from the earlier hominids. And if we put it on our evolutionary tree here, it comes at this very interesting time between two million and one million years ago where you have a proliferation of hominids, a variety of different species, and you have this big variation in the brain sizes that we have Now as we’ve also heard, this time period correlates with the radical change in human adaptation. And particularly in evidence for the precedence of animal based tissues in the diet, it also correlates again with our Turkana boy here and this general time period where you have the evolution of what we call Homo erectus. Now what’s interesting about this also is it also correlates with the time period where hominids begin to move out of Africa. So our question here is how much does this cooperative behavior that we can infer from the brain size and we can refer from the life history really affect the hominid’s ability to adapt to a variety of different habitats not only in Africa but also throughout the world? Now the only direct evidence we have for any type of cooperation comes from the side of Dmanisi and our friend here with no teeth. And I’m sure it feels very familiar with the argument that this individual must have whatever you want to argue, been taken care of. The message from this is we don’t really know how to recognize this cooperation in the fossil record. It would be very nice if we could. Now if we come back to the fossils, is there any way from the fossils we could tell what their tempo of growth is? And in this case, one of our colleagues, Chris Dean, has been working on how to infer growth and development from the formation of the teeth. I’ve been very impressed by some recent work by Chris and his colleague, Helen Livesidge in London. Not on teeth this time in particular, but on body size because what Chris has done is sort of bracketed the age of our Turkana boy at death to somewhere between seven and eight years old. Now what these graph show were really startling to me because the green one is stature. And the histogram is the stature of modern human kids who are between about 7 and 10 years old. The red line there is the estimate for the Turkana boy. It’s way in excess of the stature that you would expect. The blue is body mass or body weight. Again way outside of what you would expect in modern human kids. Now what does this tell you about the growth and development of the Turkana boy? If we go back to Barry Bogan’s figures, what it’s telling you is that the Turkana boy was growing on a trend very much more similar to the apes than to modern humans And if you look on the chart, in fact just above age there on the chimp chart is eight years old. And you’ve come out of that plateau of slow growth velocity, the individual would be then growing towards his adult weight. In humans, it would be still well within that plateau before that

growth explosion, that’s the adolescent growth spurt. Now this, to me, is very convincing evidence that the Turkana boy was not growing and developing on the human pattern. Now what that would mean is that it would reach maturity much more rapidly than if it was growing on the modern human sort of tempo and mode. Now is there anything more we can tell? Your eruption of teeth tends to still also measure how long your growth period is And what’s become very clear recently is there’s a tremendous amount of overlap not only between living human tooth eruption and tooth formation patterns and apes, but also overlap in the fossil hominids. And when it comes down to it, if you’re looking at tooth formation, your best tooth in terms of distinguishing us between the chimps and humans is your third molar. But again the take home message from this comes in the bottom graph. And the blue line there is the tooth development for your second premolar and that’s in the chimps, the red one is in humans. The vertical lines are your early Homo fossils. The green one is the Turkana boy. And although the teeth are developing at the very, very fast end of modern humans and there’s a tremendous amount of variation, it’s squarely within the ape pattern as well. And if we’re going to argue about what the life history patterns of some of these early Homo are, basically we’re talking about a much more ape-like growth and development pattern than a human-like growth and development pattern. Okay, the next questions comes in is when did we start to grow and develop as humans? And the first indication is with Homo antecessor which is about a million, not .9 million years old. The teeth are still developing more rapidly than ours, meaning they’re developing more rapidly. But the argument is made that the first ones that are closer to modern humans than to the early Homo, the early australopithecine pattern. And then as we move up to the top at about a 160,000 years ago at the Moroccan side of Jebel Irhoud and this is some of some John Jack Hublin’s and his group right there in the fourth row, where you have a very modern human growth and development pattern. Now what I want to do is sort of finish with what is probably everybody’s favorite fossil and that’s the neanderthals. And they’re a long way up the record from what we’re talking about in the evolution of the genus Homo. But I think the lessons we can learn from the neanderthal growth and development, we can usefully bring down the record and apply to some of the australopithecines. Now neanderthals of course are the alternatives to modern humans. They have brain sizes that overlap with ours. They are in the blue series of Xes there. They lived in Europe during the last ice age while anatomically modern humans were evolving elsewhere in Africa What, again, another piece of evidence that struck me that comes out of Martin Gonzales and some of his colleagues’ work from Spain is the growth and development of the little neanderthal kids. So the blue lines here are your growth from not years, up to 70 months. The horizontal lines are the equivalently aged neanderthals in relation to that modern human growth pattern. So here you can say the little neanderthals are basically plateauing much earlier than you would see in the human growth curve. Now think about being a neanderthal. I’m often happy I wasn’t born a neanderthal because, you know, what the authors are arguing here is not only do they have the energetic stress of growing and developing that large human-like brain size, they also have a serious thermoregulatory problem. Because in fact with Steve Churchlin I’ve done some modeling on the energetic requirements in neanderthals and it’s about the same as to say somebody riding the Tour de France in the Alps. It’s about 500 kilocalories a day. And you think of

these poor little neanderthals trying to support that brain and also keep warm. Now there is a second piece also that comes from other research. This is from Smith et al in 2010. And the green lines here are your expected and predicted ages for human teeth and the blue lines are the neanderthals. The take home message is their teeth are developing more rapidly so they have a more attenuated growth and development process. Now coming back to this and where you would expect to say a modern human populations to have more rapid growth and development. It’s in situations where you have a higher mortality rate. Now think about these poor neanderthals, they have a very high mortality rate, every three calistosis It’s the same as rodeo riders in the southwest. And you have these poor little stressed neanderthals with a shorter body size. Now if we come back to all of our, you know, fossil species, our australopithecines and early Homo, living in a period of very fluctuating climate in Africa, and this little zig-zag chart is just from East Africa, but also experiencing all of these environments around the world. What we really want to know is what their growth and development patterns are and how these reflect what’s happening in their lifestyle. And what I tend to think is that the same processes that affect us today were also affecting them. And this is a huge challenge, it’s very exciting research and I think as soon as we’re able to screw down into it, we’re going to find similar neanderthal-like patterns among the hominids that are living different types of lifestyles. To end, I’d like to thank CARTA. We were all having a great time here. And I’d particularly like to thank all of the attendees of a Wandergrand conference, some of you are sitting in the audience here, that we held on human biology and the origins of Homo. And a lot of these life history ideas were developed in a week-long meeting in Portugal. So thank you very much ♪ [music] ♪