When most of us picture human evolution, I imagine we all see the same image; you know the one I mean: a transformation, through a series of stages, from a knuckle-dragging caveman to an erectly striding homo sapiens! Evolution seems to happen in distinct steps, an obvious chain depicting our more primitive ancestors leading to what we see today, road map of where we’ve come from and where we are now. Hindsight is a great thing for most life, evolution is only a tiny step in the journey, with no idea of where they started or where they are going, just that they need to keep going.
Evolution isn’t something that happens to you, but to a population of you, over time, so each little step doesn’t really seem all that significant. You may think that after 200,000 years of evolution, the human race would be pretty much… done. We have managed to successfully colonise niches spanning the entire planet, from cold tundra to high mountains and beyond, and if you met a human from 65,000 years ago, they would seem pretty human. We have not seemed to change all that much. Alongside the development of modern medicine and technology, there is very little for most of us to worry about, in regard to natural selection. Modern life is a great leveller; no longer does an unfortunate roll of the genetic dice mean that you are ruled out from living full long lives and the possibility of passing on your genes to the next generation. Genetic fitness is not what it used to be. We truly have mastered this planet (for better and for worse).
Therefore, the idea that human evolution is now surplus to requirements, or at least slowed, would not be an illogical conclusion to draw. However, it turns out that the human race is still evolving, and possibly faster than ever before. So much so that the effects of evolution are being observed within a few generations, over the span of decades and centuries rather than millennia.
Recent investigations have observed an increase in the prevalence of certain anatomical features over a relatively short period of evolutionary time, this has been termed microevolution. Some examples of microevolution observed in humans include; people being born with additional bones in their feet and hands, the development of shorter jaws, and the absence of wisdom teeth. In a 2020 study conducted by Lucas et al, published in the Journal of Anatomy, they investigated the increased prevalence of another of these micro-evolutionary features, an additional blood vessel.
The median artery is found in the forearm of humans, but only usually in developing foetus, and by week eight of gestation, the median artery usually starts to recede, replaced by the radial and ulnar arteries. However in about 10% of the population in the mid-1800’s, the median artery persists, with these adults having all three arties still present in their forearms. Although this feature is becoming increasingly common.
By studying data published in medical records and by performing autopsies on people born in the mid 20th century, Lucas et al have observed a significantly greater occurrence of individuals who still have their median artery, finding it in about 30% of the population.
“This increase could have resulted from mutations of genes involved in median artery development or health problems in mothers during pregnancy, or both actually. If this trend continues, a majority of people will have median artery of the forearm by 2100.”
Dr Teghan Lucas
How and why these changes have proliferated can only really be speculated upon, but in a population of any living organism, there is clearly some mechanism present to add some genetic variation to the gene pool. This can occur through an exchange and recombination of genetic material between individuals, or by random spot mutations, such as those seen when DNA is transcribed and translated incorrectly, which may lead to a novel anatomical or physiological feature. If these mutations have no impact on an organism’s overall fitness, we would expect that the frequency that we would observe these genetic quirks within a wild population to remain almost constant. Perhaps these new quirks can be explained by the new world we find ourselves in. When we remove the selection pressures brought on by external hardships, random chance can play a greater role in which genes are passed on or if certain genetic mutations persist within a human population, otherwise known as genetic drift. Or alternatively epigenetics, define, might play a role. How certain genes are expressed might be controlled by factors outside the cell’s control. Internal body chemistry or external stimuli can affect how chromosomes are arranged within the cell, with certain chemicals either causing parts of the genome to unfurl more readily, leading to greater gene expression, or conversely cause the chromosomes to be more tightly folded leading to certain genes not being expressed as frequently.
Although the exact benefit of having this additional artery is unknown, the fact that its prevalence is increasing within the human population suggests it must do something useful. Perhaps the additional blood vessel provides better circulation to the arms, or maybe having an additional artery allows for complex surgeries to be performed, giving options to replace damaged blood vessels without the need for a donor. It is difficult to surmise just how much of a distinct advantage this particular feature would give you over your fellow humans, which would cause such a rapid rate of evolution. But we cannot see the road map, we just keep stepping forward.
Appendix- How evolved are you?
There are some features of human anatomy that we recognise as evolutionary baggage, leftover vestigial features from a previous life. I am sure we all know about the pathetic excuse we have for a tailbone and the shrivelled-up appendix, shadows of their former glory (even if the appendix is slightly more useful than we give it credit for), and due to their lack of importance, have slowly withered away. But if you are looking for a more obvious way to tell if you are more evolved than your pals, you can see if you still possess your palmaris longus. This was the tendon that would have been responsible for retracting your claws, when you were still a tree-dwelling organism. Start by making a fist and tense up your wrist and move your fist towards your body, knuckles down. Do you see a prominent ridge that runs down the middle of your arm, this is your palmaris longus tendon. If you don’t still have your palmaris longus, congratulations, you are part of approximately 15% of the population who have evolved past the need for such primitive features.
But be careful what you wish for. Reports keep coming in about how species with crablike features keep evolving, so much so the phenomenon has been given the term carcinization. When two separate species evolve similar characteristics and methods of dealing with a problem, it is known as convergent evolution, and it seems that being a crab has its advantages. So if you find yourself walking sideways and snapping at people, you may just find that life is just easier, when you’re a crab.
References:
“Recently increased prevalence of the human median artery of the forearm: A microevolutionary change” by Teghan Lucas, Jaliya Kumaratilake and Maciej Henneberg, 10 September 2020, Journal of Anatomy. DOI: 10.1111/joa.13224
Smith, H.F., 2011. The role of genetic drift in shaping modern human cranial evolution: a test using microevolutionary modeling. International journal of evolutionary biology, 2011.
Bossdorf, O., Richards, C.L. and Pigliucci, M., 2008. Epigenetics for ecologists. Ecology letters, 11(2), pp.106-115.
Heather F. Smith, William Parker, Sanet H. Kotzé, Michel Laurin. Morphological evolution of the mammalian cecum and cecal appendix. Comptes Rendus Palevol, 2017; 16 (1): 39 DOI: 10.1016/j.crpv.2016.06.001
Jonas Keiler, Christian S. Wirkner, Stefan Richter, One hundred years of carcinization – the evolution of the crab-like habitus in Anomura (Arthropoda: Crustacea), Biological Journal of the Linnean Society, Volume 121, Issue 1, 1 May 2017, Pages 200–222, https://doi.org/10.1093/biolinnean/blw031
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