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A Human Evolutionary Scenario

Here's one way of putting some of the pieces of our human puzzle together, to try to understand how we got to where we are today.  Some of the timings are backed by a combination of DNA analysis and the fossil record, but many of the most important changes, such as our sexual anomalies and our use of language, are still things to guess about. 

Scenario for human evolution according to current information

1. We diverge from other primates:  Around 9 million years ago, we branched off from our closest primate relatives, the chimpanzees, and started down a different evolutionary path. 

2. We stand up:  Around 4 or 5 million years ago, we came down out of the trees and started walking upright, probably because of changes in our habitat.  This left our hands free to carry things, particularly things we might throw.  We begin to be selected for our ability to throw things hard and accurately. 

3. Our heads begin to grow:  Around 2.4 million years ago, a gene in one of our distant ancestors mutated so as to weaken the powerful jaw muscles that enclose the skulls of all our simian relatives. This apparent disadvantage resulted in weaker and ultimately smaller jaws, but permitted the skull and brain cavity to get much larger.  Shortly after this mutation, the first members of genus Homo appeared.  (Hansell Stedman and others of the University of Pennsylvania have recently advanced this theory; see Nature, volume 428, page 415, or New Scientist volume 181 issue 2440 of 27 March 2004, page 7). 

4. Intelligence becomes increasingly important:  Larger brains proved advantageous enough that they were selected for, so human brains kept getting bigger and bigger.  This had three effects:

  • Birth became more and more dangerous as babies' skulls became larger in comparison to the size of the birth canal. 
  • Infants had a longer and longer period of dependence because their large brains needed more time to develop.
  • To ensure children's survival through the increasing period of infancy, humans shifted from the easygoing sexual patterns characteristic of our nearest genetic relatives, bonobo pygmy chimpanzees, to a monogamous, nurturing pattern more characteristic of marmosets and other New World monkeys, and of many bird species. 

5. Growing brain size creates a crisis:  Amateur evolutionary biologist Leonard Shlain has advanced the theory (in Sex, Time and Power)that around 150,000 years ago, the birth problem reached critical proportions, and so many mothers and infants were dying in childbirth in a particularly intelligent part of the population in Africa that its population declined dangerously.  The design that survived this bottle-neck of disaster turned out to be particularly viable in the long term, and included all the unusual characteristics peculiar to human sexuality: cryptic ovulation, synchronized estrus, strong female orgasm, heavy menstruation and menopause. 

6. We branch and spread geographically:  Sometime later, perhaps around 130,000 years ago, our branch of homo sapiens diverged from Neanderthals, for reasons probably having to do with differences in the ecological niches we then occupied.  An Asiatic branch also appeared.  We were all quite successful and spread across the globe. 

7. Cro-Magnons develop language and become dominant:  Around 40,000 years ago, our branch of hominids (often referred to as Cro-Magnons)  became explosively successful and spread rapidly throughout the world, replacing all other competing branches of homo sapiens, probably by killing them.  The most likely explanation for this success was the development of language in its modern form, which facilitated the kind of complex social cooperation and planning we now take for granted (this is necessarily hypothetical). 

Our prehistory, like our history, is bathed in blood

We don't like to think of ourselves as a bloodthirsty species, and indeed, even in our most warlike cultures, few humans seem to enjoy war.  All the evidence, however, points to our having evolved in a context where it was the norm for us to have to go to war in order to survive.

Von Clausewitz famously observed that war is fought over resources.  Archeologist Stephen LeBlanc has recently argued in Constant Battles: The Myth of the Peaceful, Noble Savage that human populations virtually always, in every context we know anything about, grow until they over-run available resources, at which point they must go to war to survive.

LeBlanc observed that there's a zero-sum game rationale behind this behavior.  Suppose there are two neighboring bands of chimpanzees or humans, one of which controls its population to match available resources, while the other reproduces at a rate to exceed the carrying capacity of its territory.  The band that remains in balance with available resources will have plenty to eat but will be very significantly smaller than the profligate band by the time the profligate band has grown to the point that it faces starvation.  At that point, the balanced band will find itself at war with a desparate, much larger group, and will likely be anihilated.

I believe LeBlanc is correct, both because of the wealth of evidence he assembles from evolutionary biology, anthropology, archeology and history, and because what he has to say is not particularly flattering to us as humans, which explains why previous researchers have ignored the implications of their evidence wherever possible.  Constant Battles presents a perspective that is essential to understanding our evolution, prehistory and history. 

The implications of this perspective for reproduction are important, and make an already difficult decision even harder: how do you reproduce responsibly?  The answer is by no means as simple as "just say no."  For further discussion of this issue, see our fertility and babies mean war pages. 


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