The Future Evolution of Human Populations

 I wrote a short essay for a human evolution course taught by John Hawks at Coursera. A fun and enlightening course. I cleaned-up a rather rough essay and added some references.

The Future Evolution of Human Populations

Abstract

Barring extinction, human evolution will continue. How will humans evolve in the future? Evolution is measured on a population scale and how the human population changes holds the key to how humanity may evolve. The size and structure of a population affects the amount of mutation present and the amount of mutation provides the diversity that selection must choose from. More mutation creates more chances for diversity. Here I provide four ways human populations may change in the future and how this will effect evolution.

  1. Steady State – Human population growth will slow down and perhaps contract to a sustainable size, perhaps three billion.
  2. Collapse – Global warming and environmental degradation will reduce human population to a few million.
  3. Saved by Technology – Human population will continue to grow while technology allows for a ever more crowded Earth.
  4. Escape into Space – Humans will spread out into the planets and stars with ever growing numbers.

Introduction

The World Population Clock is ticking. Today the current estimate of the world’s population is over 7.2 billion people. When one plots the curve of the growth of any biological population over time, one usually sees a logistic curve, a curve shaped like the letter S. Populations start growing very slowly to a point where they start to grow quite rapidly. Populations reach a point of saturation when resources for growth run out and population growth slows. The curve then flattens out, representing steady state. Human population started growing rapidly between 1700 and 1800 and has just started to plateau. Current estimates from the United Nations predict human population will peak at between 10 and 11 billion people. Does our planet has enough resources to sustain such a large population? Perhaps the human population needs to be reduced to 3 or 4 billion people, or even less. Human caused global warming throws another factor into the mix with some researchers (Hansen, Kharecha, Sato, Masson-Delmotte, Ackerman, Beerling, Hearty, Hoegh-Guldberg, Hsu, Parmesan, Rockstrom, Rohling, Sachs, Smith, Steffen, Van Susteren, von Schuckmann, Zachos, 2013) predicting dire consequences and collapse.

(Hawks, Wang, Cochran, Harpending, Moyzis, 2007) has suggested that rapid human population growth over the last 10,000 years has given rise to rapid adaptive evolution. The synthesis of Darwin’s theories and the theories of genetics during the 30′s and 40′s created a theoretical foundation for evolutionary change based on populations. Humans have shown a great ability to not only adapt to different environments but to change these environments to meet their needs. This has been through culture, technology and the ability to transmit this technology from generation to generation. This has also led to the expansion of human populations. But what of genetic change? The lactose mutation has spread through some of humanity but there are also cultures in which adults drink milk and don’t have the mutation. What they drink is fermented milk, fermentation being an important human technology. The theory that technology and culture have triumped genetics and that human evolution has stopped does not seem to be true. Technology and culture have led to rapid population growth which has in turn led to rapid adaptive evolution. Just how this combination of cultural change and population growth provides a feedback mechanism for rapid evolution is unclear but as we find out just what genes have changed and their affect on fitness we can perhaps better understand the future.

Scenario #1 – Steady State

If humanity can stop it’s population growth and even reduce human population to a sustainable level without major disruptions, how will this affect our evolution? For one thing, there will still be a huge human population with plenty of mutational potential. This peaceful happy population without many environmental challenges could actually lose previously adaptive regions as mutations to regions no longer selected for start to occur. This happens in lines of laboratory animals all the time (Dunbar, Wilson, Ferguson, Moran, 2007). The major problem for mankind would be losing that ability to adapt and not end up so specialized that it can’t adapt to change. The robust australopithesines are an example of a hominid branch that specialized and went extinct. Perhaps cultures would fragment and become richer. Brain evolution in particular would continue.

Scenario #2 – Collapse

Rapid and disruptive population loss could occur as the result of severe climate change. Loss of major water resources from melting glaciers, rising sea levels, whole regions becoming uninhabitable due to extreme heat and desertification, crises in agriculture, etc. Humans would face three choices:

  1. Defend small populations in scattered refugia where life would continue as before.
  2. Adapt to this hotter world.
  3. Become extinct.

Different cultures would have different solutions and different outcomes, the successful would survive. Adaption to high heat would be particularly important, not only for humans but for the web of plants and animals that help support humans. Heat stress is genetically related and adaptations around heat stress would allow humans to live in regions that had become depopulated. Huge population crashes like this are random so that those who survive must work with what variation is left. Also, what percentage of past technology and knowledge survives is also important. Populations under severe stress can lead to an uptick of the mutation rate, this is true for both plants and animals but the human rate is already slow. The major problem here is that the human population could get too small and not be able to recover.

Scenario #3 – Saved by Technology

Human population growth has always been driven by culture and technology with one feeding and driving the other. In addition, this has driven human evolution. There is the chance that technology could provide for an increasingly crowded world. The genetic variation to needed for rapid human evolution is certainly there. Humans could increasingly start living on and under the sea, underground, in larger and denser cities, cities might overrun much of the planet. This would fuel evolution. Humans would need to become less violent to adapt to crowded conditions. (Pinker, 2011) has show statistical evidence that this is actually happening, although this is hard to believe. Also, certain social skills might become more successful for a crowded world and these would be selected for. There is evidence that certain forms of human networking is under genetic selection (Fowler, Dawes, Christakis, 2009). Humans who can form bonds with greater numbers of others might perhaps have better fitness values. The present condition of culture driving population driving evolution would continue.

Scenario #4 – Escape into Space

Human beings would migrate first into space and then the near planets and eventually the stars. Like ancient hominids, modern humans would migrate in search of materials and energy. Like ancient times this would take places in waves as population pressures create a need and technologies are created to fulfill this need.  Also, like ancient hominids, populations could be isolated for thousands and even hundreds of thousands of years. Populations could become highly specialized, become extinct, become swamped and live on in the genes of later human waves. Morphological changes could become more pronounced. Gravity, either loss of gravity in space or different gravity on other planets, would change bone structure and the way a person moves around. Different and new human species are entirely possible as isolation and extreme adaption to novel environments could lead to sexual isolation, the inability of two groups to procreate. In other words, this scenario would change the evolution of the human race the most drastically. Note here that the timeline is much larger than the timeline for the other scenarios. Getting into space and living in space would be humanity’s very first challenge. Adapting to low gravity, high radiation, crowded conditions and a food and oxygen supply that would always be close to the edge would be a real challenge and the children born in space and their children would be selected for these very different and extreme conditions. The vast time it would take to to move human populations to different planets on other star systems might require that vast populations be moved in storage with either automated systems or space-adapted humans managing the flights.

Conclusion

I have presented four scenarios of human evolution based on four population scenarios. What probably will happen is a mixture of all of these scenarios as different cultures deal with challenges and different technologies are brought into play. Evolution will continue no matter what, nature will continue to select based on fitness. Humans could go extinct, they could specialize and stabilize or they could expand into different environments. Culture and technology will continue to drive human change and human evolution. I think the important thing is that now for the first time we hold our fate in our hands and the decisions we make today will profoundly affect our evolution and our future.

References

Hansen, J. et al., 2013. Assessing “Dangerous Climate Change”: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature. PLoS ONE, 8(12), p.e81648. Available at: http://dx.doi.org/10.1371/journal.pone.0081648 [Accessed March 20, 2014].
Hawks, J. et al., 2007. Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences, 104(52), pp.20753–20758. Available at: http://www.pnas.org/content/104/52/20753 [Accessed February 25, 2014].
Dunbar, H.E. et al., 2007. Aphid thermal tolerance is governed by a point mutation in bacterial symbionts. PLoS biology, 5(5), p.e96.
Pinker, 2011. The Better Angels of our Nature, New York: Viking.
Fowler, J.H., Dawes, C.T. & Christakis, N.A., 2009. Model of genetic variation in human social networks. Proceedings of the National Academy of Sciences, 106(6), pp.1720–1724. Available at: http://www.pnas.org/content/106/6/1720 [Accessed March 20, 2014].

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