A recurring philosophical question is what makes us human. It’s a good question because it appeals to our understanding of our sense of self. Part of the answer is straightforward, like how we differ from inanimate objects and even other life forms like insects or trees. What separates us from other primates on the other hand, is a more difficult to pin down.
There are a number of well supported facts—humans are generally a lot less hairy than other primates, spend far more calories in completing any given task, walk on two legs and have the power of speech. Humans also have the lowest sperm count but the largest penis size of the primates.
However, these facts whilst interesting, are more a random assortment or cluster than a coherent argument or case for what human is. Can I hope to do better? Well, I have a suggestion for you, at least from a molecular point of view.
An early view of human development was that we evolved on the savannahs of Africa. The fossil evidence is certainly consistent with Africa being the cradle of humanity, however more recently, the savannah hypothesis has been replaced by one involving water. Evidence collected since the theory was proposed by Sir Alister Hardy in 1960 suggests that humans evolved from primates who lived near water*. Not only did the water provide a convenient supply of food, but also led to our upright movement, relative loss of hair, the development of subcutaneous fat, but also our lipid intake was different.
The evidence suggests that we ate fish, mainly giant catfish, in large quantities. The fatty acid profile of triglycerides in fish, especially cold water fish, is different to that of mammals in that they have more unsaturated bonds. In particular the fatty acids known as omega-3s, a category named after the position of the double bonds (See Fig. 1), such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) were in rich supply. This was useful for brain development—an important part of how human cognition developed. We needed to be clever to follow the behaviour of animals to exploit them for our nutritional intake, to manage our intake throughout the year, but also the dexterity to catch such animals and make the best of the plant material available. This led to the development of a larger brain.
The idea that such a large brain could have developed on the savannahs seems possible and has also been supported—if humans eat the brains of other animals, for example. However, the supply of the omega-3 fatty acids is much weaker there, where that of protein is much higher. This leans away from humans and more towards musclier animals with smaller brains—indeed, animals with a similar brain/body size ratio as the prey they hunt.
The supply of omega-3 fatty acids is considerably higher in mammals in marine systems, enabling brains to enlarge. The comparisons are striking: the brain of a savannah mammal like a zebra is about 300 g, but that of a marine mammal of comparable size such as a dolphin, is about 1800g. This suggests that the supply of brain-building materials is much richer in food from marine environments. This analysis therefore supports the hypothesis that early humans developed larger brains partly because of the access to marine livestock. It is also consistent with the evidence that our brains perform better when we eat animal and especially marine animal, rather than plant, unsaturated fat.
The accumulation of omega-3 polyunsaturated fats by a land animal is therefore much more ‘human’ than one might have thought. The uniquely-human aspect may not end there—part of the grisly evidence that led to the conviction and hanging of John George Haigh, the acid bath killer, was a considerable quantity of human fat that was not destroyed by the sulfuric acid he used. Even with primitive techniques it was possible to identify it.
This analysis has one other intriguing possibility. If correct, it suggests that humans moved from in-land areas towards water, and whilst there was some contact with it, we remained land-based animals. Did any such primates develop further, or even leave the land altogether?
We may never know for certain, and we have more answers and more questions as a result of this work. We also have a better idea of where we might fit in the world and can therefore infer what it is to be human from the evidence of the environment that may have produced us.
References and further reading
*Much of the background to these ideas is explained beautifully in a radio programme by Sir David Attenborough that can be found at http://www.bbc.co.uk/programmes/b07v2ysg . This programme is also a great joy for a lipid scientist pedant as the distinction between lipid and fat is made clear, as is even the type of fatty acid.