Presentation for a Panel discussion on the topic "How unique, in fact, is the animal man? (Hur unik är djuret människa, egentligen?)" in 2008 at Göteborg Book Fair, Gothenburg, Sweden] Published 4/25/2011

Reproduction strategy as a driving force for Human Evolution


Summary: Humans form couples and are sexually active without long interruptions. This is an important difference between humans and the great apes. Thus, humans produce more children than they can easily handle. Yet, humans have a strong desire to take good care of their children. This creates a conflict. In this paper, we will show that quite a bit of human evolution can be understood as a consequence of this one conflict, and the attempts to resolve it.




One usually distinguishes between two strategies of reproduction among animals, the R-strategy and the K-strategy.


The R-strategy is to produce a very large number of offspring, but not take much care of them. Because of the large numbers, some offspring will survive, and the species survives. This strategy is represented i.e. by some shrimps.

The K-strategy is to produce a small number of offspring but to take well care of them, so that they survive. This strategy is represented i.e. by the great apes, our closest relatives among animals. After having a baby, the female chimpanzee will carry it around for several years. She will not be sexually active and have another baby until the first one can manage pretty well on its own.


Humans have a reproduction strategy which falls between R and K and which creates a conflict. In the way humans form couples and are sexually active without long interruptions, they will produce more children than they can easily handle. Yet, there is a strong desire among humans to take good care of their children. Quite a bit of human evolution can be understood as a consequence of this one conflict and the attempts to resolve it.


Some important and natural strategies to try to resolve the problem of having many children and still take good care of them are

(1) To colonize new regions to accomodate an increasing population.

(2) To develop new technology

(3) To develop new social structures

(2) and (3) will increase the capacity of existing habitats to accomodate larger populations.


There is abundant evidence, that humans over the last 2-3 million years have adopted the strategies (1)-(3). One can expect that this leads to a biological and social evolution according to (A),(B) and (C) below.


A/ One can expect natural selection to favor ability and willingness to adopt the strategies (1), (2) and (3).


If just colonization (1) is used, one can expect that a population in a region will increase to a larger population in a larger region, which will increase to an even larger population in an even larger region etc. This is "exponential growth".

Thus we have

B/ One can expect a fast colonization of all regions which are habitable and can be reached by existing technology.


If the development of new technology (2) or new social structures (3) is used in some region one can initially expect a local population expansion in that region.

But the capacity of that region will soon be reached and there will be pressure to either develop more new technology or new social structures, or, to move into adjacent regions, or both. The pressure to move into adjacent regions creates a pressure to use the new technology/social structures also in the adjacent regions to accomodate the population increase, and so the new technology/social structures quickly spread to the whole human population.


Thus we have

C/ One can expect the following feedback loop:

New technology/social structures give a local population increase and the one way to handle this population increase is to use more of the new technology/social structures. So, the new technology/social structures will spread fast through the entire human population. This feedback loop will be combined with A/ and B/.


To test this model, we will consider the evidence that A/,B/ and C/ have actually happened.


Brain size and intelligence,  complicated social structures and willingness to colonize  new regions now constitute differences between humans and the great apes. So, there is strong evidence that A/ has happened.


In the relatively recent past we have some examples feedback loops represented by (B) and (C). When farming was introduced there was a local population increase. The way to handle this increase was to introduce farming in more regions which led to more population increases in those regions. So, by this feedback loop, farming quickly spread to the whole world. And after having been hunters-gatherers for a few million years, humans became farmers in a few thousand years.


The introduction of farming led both to increased population densities and to colonization of new regions. But the introduction of farming did not resolve the conflict between having many children and having a desire to take well care of them.


In an even more recent past, a few hundred years ago, the industrial revolution started. Initially, it represented development of new technology. But soon it gave rise to new types of feedback loops in (2) and (3) like: With new technology, more people can be educated and develop more new technology. This feedback loop leads to an accelerated development of new technology.


Today we are in the midst of such a feedback loop. And from having been farmers for a few thousand years, humans have adopted a modern lifestyle in a few hundred years. And much technology, that is a few decades old, is now obsolete.


This development has also done more to increase population densities than to colonize new areas. And the development seems to also gradually resolve the millions of years old conflict of having more children than can comfortably be taken care of. In a growing number of countries today, the reproduction rate is below 2 children per individual.


It is natural to consider whether we can find feedback loops like (B) and (C) in a more distant past, and what they may have been like. One obvious case to consider is the early expansion out of Africa, that started approx. 2 million years ago. The scant archeological evidence seems to be consistent with the idea that there was a fairly fast colonization of large areas outside of Africa. And there seems to be growing evidence that some impotant technology, like Acheulean handaxes, spread fast through the human population. And there is evidence that there was also increased brain size.


Was this expansion connected to the ability to use fire systematically? Presently, there seems to be no consensus among researchers on this. Some researchers believe that the systematic use of fire is approx. 2 million years old, and there is some evidence for that. Others believe that it is much more recent, perhaps approx. 400000 years old, and there is evidence for that, too.


For an even more distant past one can consider how the introduction of stone tools affected regions of habitat for early humans, more than 2 million years ago.


In conclusion - we see that there is strong evidence that A/, B/ and C/ have happened in the past. Further research may decide whether it happened to a larger extent than is known today.


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