A recurring philosophical question is what makes us human.\u00a0 It\u2019s a good question because it appeals to our understanding of our sense of self.\u00a0 Part of the answer is straightforward, like how we differ from inanimate objects and even other life forms like insects or trees.\u00a0 What separates us from other primates on the other hand, is a more difficult to pin down.<\/p>\n
There are a number of well supported facts\u2014humans 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.\u00a0 Humans also have the lowest sperm count but the largest penis size of the primates.<\/p>\n
However, these facts whilst interesting, are more a random assortment or cluster than a coherent argument or case for what human is.\u00a0 Can I hope to do better?\u00a0 Well, I have a suggestion for you, at least from a molecular point of view.<\/p>\n
An early view of human development was that we evolved on the savannahs of Africa.\u00a0 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is certainly consistent with Africa being the cradle of humanity, however more recently, the savannah hypothesis has been replaced by one involving water.\u00a0 Evidence collected since the theory was proposed by Sir Alister Hardy in 1960 suggests that humans evolved from primates who lived near water*.\u00a0 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.<\/p>\n
The evidence suggests that we ate fish, mainly giant catfish, in large quantities.\u00a0 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.\u00a0 In particular the fatty acids known as omega-3s, a category named after the position of the double bonds (See Fig. 1<\/em>), such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) were in rich supply.\u00a0 This was useful for brain development\u2014an important part of how human cognition developed.\u00a0 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.\u00a0 This led to the development of a larger brain.<\/p>\n The idea that such a large brain could have developed on the savannahs seems possible and has also been supported\u2014if humans eat the brains of other animals, for example.\u00a0 However, the supply of the omega-3 fatty acids is much weaker there, where that of protein is much higher.\u00a0 This leans away from humans and more towards musclier animals with smaller brains\u2014indeed, animals with a similar brain\/body size ratio as the prey they hunt.<\/p>\n Figure 1.\u00a0 Docosahexaenoic acid, DHA.\u00a0 This fatty acid has twenty carbons and six cis-double bonds.\u00a0 The names given to the carbons are marked.\u00a0 The fatty acids referred to as \u2018omega-3s\u2019 are ones with a double bond at that position (includes DHA).\u00a0 The fatty acids referred to as omega-6s do not have a double bond at this position (includes linoleic acid).<\/p><\/div>\n The supply of omega-3 fatty acids is considerably higher in mammals in marine systems, enabling brains to enlarge.\u00a0 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.\u00a0 This suggests that the supply of brain-building materials is much richer in food from marine environments.\u00a0 This analysis therefore supports the hypothesis that early humans developed larger brains partly because of the access to marine livestock.\u00a0 It is also consistent with the evidence<\/a> that our brains perform better when we eat animal and especially marine animal, rather than plant, unsaturated fat.<\/p>\n The accumulation of omega-3 polyunsaturated fats by a land animal is therefore much more \u2018human\u2019 than one might have thought.\u00a0 The uniquely-human aspect may not end there\u2014part 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.\u00a0 Even with primitive techniques it was possible to identify it.<\/p>\n This analysis has one other intriguing possibility.\u00a0 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.\u00a0 Did any such primates develop further, or even leave the land altogether?<\/p>\n We may never know for certain, and we have more answers and more questions as a result of this work.\u00a0 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.<\/p>\n <\/p>\n <\/p>\n References and further reading<\/strong><\/p>\n \u00a0<\/strong>*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<\/a> .\u00a0 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.<\/p>\n <\/p>\n![](\"https:\/\/i0.wp.com\/www.samuelfurse.com\/wp-content\/uploads\/2016\/12\/Labelled-DHA.jpg?resize=720%2C185\")
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