Human prehistory, -behaviour, etc. – some general remarks
I recently left a comment on another blog (it happens quite rarely these days), and after thinking a bit about it I decided to reproduce the comment here and add some more details. It seems that I already take a lot of the knowledge I’ve only quite recently acquired for granted, and that I’m sort of implicitly assuming other people know this stuff too. I think it’s quite interesting that I do this, but the post will not be about that angle – I may explore that later – but rather it will be a post with a few important nuggets of information about what the human past looks like. First, the comment (but do read the post as well) – the first paragraph is a quote from the original post, the rest is my response:
“”I venture to say that we are biologically compelled to hold these attitudes. Evolutionarily speaking, it would be a desirable strategy to mate with people who possess very different genes, in order to ensure genetic diversity and to improve your offspring’s chances of survival. But mating a partner significantly older than yourself increases the health risks of your children, and should therefore be discouraged. These evolutionary-friendly instincts are so deeply ingrained that couples in May/December relationships who do not want children nevertheless still face criticisms just by virtue of the age differences.”
This sounds like a just-so story to me. One could look for cross-country data, but a simpler way to figure out what’s going on is, I think, to just go back in time and see if ageism was always as widespread as it is now (you’d expect that if biology/genes is driving behaviour). This is most certainly not the case. In most societies in the past, age differences have mattered much less than they do in modern societies today; for example, in ancient Greece the ‘usual marriage’ seem to have been between a female at the age of 10-15 and a male twice that age. What we might now consider child brides were common in many prehistorical societies across the world, because the alternative – waiting to have children until the age where people have children today (close to 30 in Denmark) – was not an option because of very high mortality rates (if females didn’t start have children relatively early in their lives, the populations would go extinct).
I also really don’t see why age discrimination would necessarily be adaptive, given that a) most people in prehistoric societies never got old enough to live to experience age-related infertility and b) most of the period of human history where selection has taken place has been a period where humans, both males and females, have had relatively small avalable mating pools. High mortality rates combined with low mobility and low population densities made sure that there were never a lot of potential partners around. These days mating pools are enormous, and so people react the way you’d expect, by becoming increasingly more critical of the partners which are available to them. If you have a lot of options, you can afford to be picky, and I think this is a main driver of ageism and similar effects. Ageism was probably unheard of 10.000 years ago, and I really doubt it had much influence on partner choices 150.000 years ago.
It has generally been the case that in societies which have obtained a sufficient level of organizational complexity to display non-irrelevant income variation across individuals (i.e. societies with surplus food), older males have had an advantage compared to younger males in the mating game, because they had had more time to gather ressources. And so, generally speaking, males have been older than their female partners and the more ressources you would be able to gather in an additional year as a non-mating male, to some degree the larger you’d expect the acceptable age-gap between partners-to-be (ceteris paribus).”
A very general notion here is that the world we live in now is very different from the world your ancestors lived in. Most people don’t think much about the changes that have taken place because they don’t know the ways in which the past was different in any amount of detail (they probably know that people who lived in the Neolithic didn’t have airplanes, but…), but that also means that they will sometimes make assumptions which are flat out wrong about the way the past ‘worked’. I touch upon some of the ways people might be wrong in the comment above, but I thought I should make some more general comments here:
First, a note on human population and population densities in the past. The number of humans pre-farming was, compared to today, very low. One estimate of the total world human population just 12.000 years ago: 1 million individuals. In the entire world. These 1 million people lived in areas as far apart from each other as Australia, Tierra del Fuego in South America, Japan, southern Europe, South Africa… Pre-farming, the carrying capacities of the areas humans inhabited were, to put it mildly, not very high – because it actually takes a lot of calories to keep a human alive. There weren’t a lot of people around, so you couldn’t afford to be all that picky – neither when it came to what to put into your mouth, nor when it came to whom to have children with. An interesting note here is that John Hawks some time age pointed something out to me which I did not know/had not thought about: That many prehistoric human females were either pregnant or lactating most of their adult lives. This is likely part of the reason why females are smaller than males and why their basal metabolic rate is lower than the basal metabolic rate of males; when you add the extra calories needed to carry a child to term or feeding it afterwards, the energy expenditure of females probably weren’t all that different from the energy requirements of males. They needed roughly the same amount of calories but used the calories differently.
Most people today live in areas where the population density is far, far higher than anything prehistoric humans could have even imagined. This factor relates, as I mention in my comments, to mating behaviour – but it relates to a lot of other behaviour too. For example, the food procurement strategies prehistoric humans made use of meant that the threat of exclusion from the tribe was a much, much more serious threat in the past than it is today; if you were kicked out of the band, in all likelihood you’d simply face starvation and death. Other factors which are to some extent contingent upon population density are ‘ingroup-outgroup dynamics’ (if there are no other tribes around nearby, intratribal competition dominates intertribal competition), ‘societal complexity’ (closely related to degree of specialization), and ‘speed of transmission and likelihood of fixation’ (this goes for both technological innovation and for genetics – if both population density and mobility are lowered, then both transmission speed and arguably also the likelihood that substitution will actually take place will also be lowered, at least unless the effect size is very significant).
I’ve touched upon this one before as well, but it’s important because people often get the scales wrong. Human (pre-)history is much longer than most people probably think, and it looks quite a bit different from what most people think. The earliest bipedal hominins emerge in the fossil record somewhere around 6-8 million years ago. The human past looks nothing like a tree with branches, unless the tree looks quite different from most trees; you need to include several branches which overlap and some twigs which jump back to other branches, or perhaps even the trunk. I recommend this bloggingheads episode with Razib Khan and Milford Wolpoff if you want a much more detailed treatment of this subject. Here I’ll just list the species in the table of early hominins in chapter 2 of THP and add some general remarks. The list: Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus kadabba, Ardipithecus ramidus, Ardipithecus anamensis, Ardipithecus afarensis, Ardipithecus garhi, Ardipithecus africanus, Australopithecus (Paranthropus) aethiopicus, Australopithecus (Paranthropus) boisei, Australopithecus (Paranthropus) robustus, Homo sp. indet. (possible early Homo, indeterminate species), Homo habilis, Homo rudolfensis. Many of these species display some level of overlap with another one of them – some of them were around simultaneously. These species had all died out or evolved into different species by 1 million years ago, most of them much before that. There were a lot of branches on the ‘human evolutionary tree’, and many of them died out along the way. We have left a lot of failed experiments behind us in the past. An interesting notion here is that we’re not even completely sure in all cases which species actually died out and which ones instead evolved into the other guys we’ve found to have been around later on – making that distinction sometimes requires a bit of educated guesswork. The farther back in time you go, the murkier it gets.
The cranial capacities of the listed species are often unknown, particularly when it comes to the oldest lineages, but an estimate (based on one specimen) of the cranial capacity of Sahelanthropus tchadensis, a(n assumed bipedal) species estimated to have been around somewhere between 7-6 million years ago, pegs it at around 320-380 cc. This is not different from the brain size of chimps. Most of the australopithecines for which data exists are also in the 400-550 cc neighbourhood. Brain size increased over time, Homo rudolfensis, which was around from perhaps as early as 2,3 million years ago (this is contested) to around 1,8 million years ago, had a brain volume of 600-800 cc – around 55-60% of that of an average modern human. We separated from the chimps a very long time before we ever got that far – and H. rudolfensis didn’t exactly do a lot of cool stuff with that extra brain power.
Now, I think it’s important to note here that if you take human (pre-)history to be the time period from when we split apart from the other apes and then up to the present point in time, these species just mentioned make up most of human history. And the fact of the matter is that most of them were not very smart – as in, most of the first ones were not significantly smarter than chimps. Around 2,5 million years ago some of them started making stone tools. But aside from primitive stone tools – well, that’s pretty much it. They haven’t left anything more complicated than that behind, something that the few people I’ve talked about this with find to be somewhat surprising. Stone tools. That’s it. Then again: “Clearly, much of the material culture of early hominins would have been made from organic, perishable materials such as wood, bark, and horn, or would have made use of unmodified materials that would leave little if any visible traces in the archaeological record to identify them as tools.” (THP, p.66) So it’s hard to tell just how advanced they actually were. But even so, it remains the case that for hundreds of thousands of years the only sort of ‘technological progress’ we can track relates to how these early humans applied their increasingly larger brains to making better stone tools. Mind you, the making of stone tools was a good idea and there was quite a bit of progress along the way. Stone tools made hunting more efficient. And they got better at making them over time. But still, this isn’t exactly super impressive, is it?
Brain size increased quite a bit over time, but it was a gradual process – the history of human evolution is to a large extent the history of the increasing encephalization of a species and the consequences of that encephalization. And the combination of two important observations here makes for another important point, one I’ve mentioned before: Human brains evolved to the ridiculously large size it is today over a very long period of time, but we had huge brains for a very long time before we started to actually ‘doing much with it’. Homo heidelbergensis, which lived from 600.000-400.000 years ago, had an estimated cranial capacity of 1225-1300 cc, not significantly different from the cranial capacity of modern humans (even though the cranium looked a bit different; for example they had quite large browridges). That’s around half a million years ago. These guys had huge brains, but behaviour was extraordinarily primitive. What we usually associate with modern human behaviour didn’t become firmly established until around 50.000 years ago. Stuff like the construction of dwelling structures; burials; textiles; the making of baskets to store stuff. Much of it comes much later than that. First art? Around maybe 65.000 years ago.
This is the point where you realize how important culture is, when it comes to behaviour. How important it is that you are able to share your knowledge with others, perhaps even with people who are no longer alive. How important it is that you don’t need to get all the good ideas yourself and how many ideas you take for granted without thinking about them. Ideas like: ‘if I build a shelter to protect me from the rain, I will not get as cold and wet when it rains.’ This is actually an important idea, and it took a lot of time for people with brains roughly the same size as yours to get it – like, it took them many tens of thousands of years!
You are the descendant of those people. Their brains had roughly the same potential as your brain has. This is a humbling thought. It’s easy to think that they were just incredibly stupid, but the truth of the matter is probably rather that they just didn’t know any better. And if you had been born at that time, you probably wouldn’t have known any better. I started out this monologue by saying that the world we live in now is very different from the world your ancestors lived in. This is true. But it’s worth having in mind also, in particular when you think about ‘what was being selected for’ over the course of human evolution, that even if you limit your analysis to anatomically modern humans of the Sapiens variety, the humans themselves were different back then. They were very much like us, but also so very different. Despite the name we’ve given ourselves, for a long time we weren’t very wise (some current members of our species would surely argue that we still are not).