November 26, 2020

I, Science

The science magazine of Imperial College

Perplexing, isn’t it? Homo sapiens, as a species, have made it. There are now over 7 billion of us swarming across the planet ...


Perplexing, isn’t it? Homo sapiens, as a species, have made it. There are now over 7 billion of us swarming across the planet, going about our daily business and often forgetting our humble origins. Indeed, often forgetting that we are a bunch of bald apes with enormous heads, doing weird things like watching football matches and hoovering and writing blogs. If evolution is all about survival of the fittest, where those organisms with the greatest reproductive success increase their gene frequencies and are ‘evolutionarily successful’ then why aren’t we all focussing on just having sex? Let’s face it; something incredibly weird has happened to human evolution.

Only 0.1% of the human genome exhibits variation, in other words we are 99.9% genetically identical. Compare any two humans, ANY two humans, and they are vastly more similar genetically than, say, a Western and Central African Chimpanzee. Despite this, ever mounting datasets from projects such as the 1000 genomes project and the HapMap project allow scientists to unpick signatures in our genetic ancestry, and identify startlingly recent instances of human evolution.

Between 8,500 and 2,500 years ago, humans started behaving differently. Many were abandoning their hunter-gatherer way of life for a more sedentary one in which they had fixed homes and farmed the land. This period is known as the Neolithic revolution. Changes in culture during the Neolithic revolution hugely altered the selective pressures which would guide the natural selection of human individuals.

Many adaptive genes increased in frequency during the Neolithic revolution, due to the drastic changes in diet. AMY1 gene, for example, currently exists in the human genome at between 2 and 15 copies, depending on the individual. AMY1 is a gene which codes for amylase, an enzyme that breaks down starchy food like bread into glucose, which can then be used to fuel respiration. So if you have a higher copy number of AMY1 in your genome, you can break down proportionately more starch. Interestingly, individuals with a higher copy number AMY1 are associated with regions where starchy food was (and still is) a key staple of the diet. So in Japan, Europe and the USA, where starch has historically been essential to the diet, AMY1 levels are higher, whereas the Mbuti and Biaka people of Africa (who live primarily by hunting and gathering) have comparatively lower AMY1 copy number.

As society has progressed and western culture has emerged, diet has not only changed but also our living standards. It is safe to say that in western culture many people are obsessed with cleanliness and as such alienate themselves from parasites and bacteria. For natural selection to be directive, a selective pressure needs to be present so if many humans live in a very hygienic environment, there is little evolutionary incentive for the species to continue to evolve immune defences to parasites and bacteria.

This ‘Hygiene Hypothesis’ goes further and says that intense cleanliness has meant that humans are slowly losing some of their ability to respond to infection. And this sounds feasible, since in this day and age even if you are genetically unhealthy, you have a plethora of antibiotics available, and so may still live to a ripe old age and have babies. Infectious diseases have fallen since the advent of medicine but immune disorders such as Coeliac disease, asthma and Crohn’s disease are on the rise. The Hygiene Hypothesis suggests that as we move beyond how our lives were in the ancestral state, we become increasingly ill-adapted to our environment.

The incredibly debilitating disorder Crohn’s disease, where patients experience life-long abdominal pain, vomiting and diarrhoea, provides a fascinating example of how our pre-historic environment once supplemented disease. In this modern age, those with Crohn’s disease are at the whim of modern medicine. Interestingly, treatment with an age-old parasite, the tapeworm, alleviates many of the symptoms of Crohn’s.

The parasites and bacteria our ancestors faced not only helped them develop, but drove the evolution of human resistance to infectious disease. To some extent, this aspect of evolution may be stalling.

So great! We have definitely evolved in the last 8,500 years. But are we evolving NOW? The inevitable and slightly boring answer is a resounding yes. Evolution is defined as the change in gene variant frequency over time within a population. Given that we all have genes, and some of us are having children, evolution must be occurring. The more interesting question is: in which direction is evolution taking us, if any?

We have established that a selection pressure imposed by the environment drives the direction of natural selection and thus morphs the future population. So, what can we say of our selective pressures?

Well in fact, more than any organism ever on this planet, humans are now creating their own selective pressures.

Here I would like to introduce the concept of the ‘Extended Phenotype’, a beautiful concept developed by Richard Dawkins. A phenotype is an observable trait of an organism. So my red hair, or your height or my brothers blood type, are all phenotypes. The extended phenotype takes a step further. Not only is my red hair a phenotype (incidentally; dyed red hair, but then again did not my genes, development and environment not compel me to decide to dye it red?) but so too is anything that I build or create. Just as much as a bee’s hive, or a beaver’s dam, is an extended phenotype, so too is human society, built by and lived in by humans.

So why am I talking about this? I am trying to point out a fascinating and also incredibly worrying loop in the course of our own evolution. As our extended phenotype becomes ever more convoluted and complex, it extensively defines our environment. So much so that we are in fact now being subject to the selective pressures that we ourselves have created. As more and more human generations are born, the very fabric of our nature weakens. Our biology becomes hyper-dependent on our environment, an environment which we (the products of our biology and environment) have ourselves created.

So what for the future of our species?

One thing is for sure: our species is changing. In particular, as our society becomes ever more complex, it is perhaps also becoming increasingly fragile. As we become adapted to an ever more artificial environment, we can only become more vulnerable – and what will happen when that artificial environment ceases to exist? Only time will tell.