What makes us human?

What is it that separates us from our animal cousins and makes us essentially human? While our closest animal relatives have been swinging from the trees, humans have built a world that is vastly more complex. What are the defining features that have allowed us to do this? Are we special, and separate from all other animals, or are we just an especially successful mammal? To understand our past and to plan for our future, we need to know what it is that makes us human. There are many ways in which we differ from other species, some trivial, some important. Here are some for the important ones …

Population size

Humans are by far the most common primate on the planet. There are about 6500 million of us spread across the globe, inhabiting just about every available environmental niche.

Unlike most other animals, humans can tolerate such a climate range because they carry their own tropical micro-environment with them in the form of clothes and dwellings. We have inherited our need for warmth from the common ancestor of apes and humans that evolved in the tropics.

Genetics

Although we humans pride ourselves on our individuality, genetically speaking we are boringly samey. We are a genetically uniform species.

The average genetic difference between two randomly chosen humans from the same place is less than the genetic difference between two chimpanzees. In all probability, this is because humans are a much younger species than chimps and we haven't been evolving for so long.

Skin

Humans can claim exclusivity among apes in having a special type of sweat gland – more than a million of them, in fact, all over our bodies. On the hair front we are singularly lacking. We have pigmented skins, even those of us with lighter skins retain the ability to become tanned.

These characteristics are thought to have evolved in response to heat stress caused by running and walking on the scorching plains. Early human ancestors in Africa were probably bald, pigmented and very sweaty!

Posture

Humans and birds walk exclusively on two legs while apes and lizards are occasional uprights. We have a whole bundle of unique skeletal, muscular and physiological adaptations for upright travel.

The advantage of being two-legged is that we have our hands free for carrying and handling things whilst walking or running – an aptitude that facilitates gathering food and hunting with tools. Although that may not have been why we evolved an upright posture in the first instance.

Reproductive patterns

Human females are continuously receptive to the sexual advances of males, unlike most female apes who vary their receptivity by coming in and out of 'heat'. The interval between births tends to be shorter in human hunter-gatherers than it is in apes.

Both human and ape females stop having offspring at around the same age – at about 40. Since humans can live for nearly twice as long as most apes, women have an unusually long post-reproductive phase in their lives. We are the only animal to have evolved long-lived grannies!

Big heads

Our brains are at least three times as big as that of an ape of a similar body size. The jury is still out on whether differences in brain size between humans really reflect differences in intelligence: there is still no proof that bigger means smarter. It is quite possible that more complex brains beget brighter people.

Nevertheless, the evolutionary drive to the human bighead does seem to have endowed us as a species with enough brain to learn much more intricate skills than other animals. We also have unique neural centres for specialised skills, like the use of speech and grammar.

Language

The ability to talk is arguably the single most important feature that makes us human. Language allows us to employ symbols and symbolic thinking in a way that we take for granted.

With language we can share plans. We can share a plan for the future. This ability underlies the rules that govern our alliances, marriages and kin relations. It makes possible propaganda, art, the division of labour, strategically planned warfare and the socialisation of the population at large. It allows human behaviour to be governed by the complex and subtle rules that we know collectively as human culture.

Social lives

Many primates have intricate social relationships, but among apes, we lead uniquely complex social lives. Apes and humans share an ability to understand that others have beliefs, desires and intentions that are different from our own. Known as a theory of mind, this capability extends to many more levels of relationship in humans than in does in other primates.

In human societies, kinship and friendship networks usually extend throughout any populations speaking the same language. In our modern world, this means that our relationships are not limited to physical proximity as they are in other animal societies.

Technology

Some of the apes are famous for tool use, but none make tools which consist of more than one working part. Humans are the only animal to have mastered technology.

At its most basic level we use technology to feed, house, clothe and transport ourselves. With it we can contain ourselves within our own tropical micro-environments and inhabit every inhospitable corner of the globe.

Back to the future

We are probably the only species on the planet to contemplate its own origins and its own future. By looking back over evolutionary time, by understanding the stuff we're made from, we might be able to plan what we need for the future. Our planet cannot last forever because the sun only has a limited life time before it burns us all to a frazzle.

Will humans be the first animal species from Earth to develop the technologies to take the micro-environment that they need into space? Might sound far-fetched but ultimately this is what our species will have to do if it is to survive.

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