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September 2006

Feature

 

Human evolution: the evidence all around us

Dr. Rebecca Rogers Ackermann

September celebrates "African Origins" in Africa. While most people associate African Origins with information obtained by studying fossil records of human evolution, this article takes a different perspective 
by looking at the evidence for evolution that we see all around us in ourselves.

Photo credit: Kurt Ackermann

Evolution is the fundamental underpinning concept of biology. It has shaped and continues to shape the diversity of life around us, from elephants to insects, fynbos to ostriches. It has also, of course, shaped human diversity. But what exactly is evolution? In a nutshell, evolution is change through time. This can be change over a very short time (for example the virus that causes AIDS has flourished in large part because of its ability to evolve very quickly), or over very long periods of time ('living fossils' like horseshoe crabs have changed very little over many millions of years). 

Our own evolutionary history, since our ancestors diverged from those of the other primates, extends back around six million years. Evidence for this evolution comes in many forms. Of course, there are the fossils. We have a rich fossil heritage in Africa, much of which comes from South Africa, from very early human ancestors who lived in the Sterkfontein Valley around 3 to 2 million years ago, to modern people who looked essentially like us and inhabited the Cape 100,000 years ago. But fossils are not the only evidence. 

Examples of human evolution are all around us, and we can learn a lot about the past by looking at things in the present. For example, traces of our evolution can be found in what we look like - in things like the shape of our skulls or the color of our skin. We can gain insight into evolution by examining our diet - what we can and cannot eat, as well as what we must eat. 

We can see evolution in the diseases we get and the organisms that share our world. And perhaps most clearly, we can see it in our DNA. Here I will highlight just a few of the many well-known examples from our modern world that give us insight into evolution that has occurred in both the distant past, and the relatively recent past, and will briefly touch on how evolution is still occurring today.

Why do we need Vitamin C in our diet?

This is a good example of how a disease we see today results directly from evolution occurring in the very distant past. It is well-known that humans must eat lots of vitamin C to stay healthy. Vitamin C is a nutrient found in many fresh fruits and vegetables, particularly the citrus fruits. Without it, humans develop a terrible disease called scurvy, which is one of the most serious diseases affecting teenagers today, causing bleeding gums, bruises, and even death. Most other mammals (like your dog) synthesize their own vitamin C, and therefore don't get scurvy. Why do we get it? Because our bodies do not make vitamin C (ascorbic acid). We have the same genes for vitamin C production as other mammals, but a frame-shift mutation has made one of these genes non-functional [1].

But we are not alone in having this mutation - it is a trait we share with other primates [2]. This tells us that this mutation likely occurred in the distant past, perhaps even in the ancestor of all primates sometime around 70 - 80 million years ago. What is most curious is that this mutation didn't get weeded out of the gene pool of those early primate ancestors by the evolutionary force of natural selection. But there is a good reason for that, as early primates (like most primates today) lived in tropical regions and ate lots of fruits (e.g. they were largely frugiverous), and therefore the mutation was not lethal to these animals and was passed on to all descendents. Only in relatively recent times has this lack of the ability to synthesize vitamin C became a problem for humans, who moved out of tropical environments, developed agriculture, started traveling on ships, and eating samoosas and pies (no fresh fruits or veg). That's when we started getting sick. Feed our closest primate relatives such a diet and they get sick, too!

Why does milk make so many people sick?

Figure caption:
Seventeenth century ink and wash drawing of a Khoikhoi cow being milked, from The Khoikhoi at the Cape of Good Hope, (1993) published by the South African Library, with text by Andrew Smith and Roy Heiffer.

The inability to digest milk (lactase deficiency) is normal in mammals, including humans. Following infancy (around 2 - 5 years for humans), the ability to produce the lactase enzyme, and therefore digest milk sugars (lactose), is lost [3]. This is often termed lactose intolerance, a condition which leads to excess gas production and often diarrhea. Interestingly, although most human populations can't digest milk (90% of the world, or more - see discussion in [4]), a quick survey of a room full of adults in South Africa will show that many more are able to drink milk than we might expect. Why is this? In some populations that have a long history of dairy farming, the ability to digest milk has been retained into adulthood, through the evolutionary force of natural selection, as milk is a major nutritional source for these people. This is the case for the herders who inhabited regions of South Africa for much of the Holocene - the last 10,000 years. It is also true for other groups in Africa such as the Fulani of east Africa, and many northern European populations.. Therefore, the fact that many South African adults can digest milk tells us something about the recent evolutionary history of South Africans - that many of us had ancestors, whether from Africa or Europe or other places - who were pastoralists.

Why is human skin color so variable?

Skin color is one of the most obvious ways in which humans differ from each other. But we were not always this different from each other, and in fact the range of skin colors has evolved relatively recently, and is closely tied with two things - the loss of body hair and migration out of Africa - and the selective pressures that were put on our skin by both. Skin color is distributed in indigenous peoples so that darker skins are near regions of high ultraviolet (UV) radiation from the sun (i.e. the equator) and lighter are regions of low UV radiation, and there is a gradation of skin color between. Why is this so? 

The earliest members of our lineage (before about 2 million years ago) lived in Africa. Initially they probably had body hair which protected them from the sun, like the other apes do, and had relatively fair skin under that hair. But when human ancestors lost that body hair, in part so they could be cooler in the hot African savannah environment, darkly pigmented skin evolved. Melanin - our skin pigment - is a natural sunscreen, and dark skin evolved through natural selection to protect skin and sweat glands from injury caused by the sun (like skin cancer), and to protect against the destruction of nutrients like folate in our bodies from UV radiation [5,6]. 

But if dark skin is so beneficial, then why did light skin evolve? When human ancestors migrated outside of Africa relatively recently, certainly within the last million years, varying degrees of light skin evolved in order to permit absorption of sunlight in low UV regions. Sunlight stimulates the synthesis of vitamin D in our bodies, preventing vitamin D deficiency and the diseases (rickets, osteoporosis) that come from it [5,6]. So the range of skin coloration we see today represents essentially a selective tension between the need for sun protection and for vitamin D synthesis, which has resulted from our widespread colonization of the planet.

Are humans still evolving?

The previous example shows us that skin color has changed more than once in human evolution (from light to dark to mixed), which indicates that this is a very adaptive, labile trait [5], and therefore could easily change again in the future under changing environmental conditions and new selective pressures. Another factor affecting the distribution of skin color today is our ability to move anywhere quickly (using airplanes and the like), which clearly allows for a lot more mixing-up than in the distant past. 

Who knows, our descendents may all soon be the same medium brown color! But this example also brings up another important point -- that our modern culture has the ability to affect how evolution is working on us. In the case of skin color, we can alleviate the selective pressures by wearing clothing or using sunblock (for light skin in equatorial regions) or by taking vitamin D supplements (for dark skin in very northerly regions). 

This influence of culture has often led to the erroneous idea that evolution has stopped in humans. In fact, we are still evolving, and will undoubtedly continue to evolve into the future. Some of the best evidence we have for current evolution is tied to disease susceptibility and/or resistance (such as the balancing selection that is maintaining sickle-cell anemia in some African regions because it offers protection against malaria), and there is a lot of current research on identifying the selective pressures shaping modern humans (for a good recent summary of this issue, see [7]). 

Finally, it is also important to remember that because our evolution takes place in the context of a specific environment (and can change if the environment changes), the extreme impact humans are having on our environment will also undoubtedly affect the future evolution of our species, possibly even in our lifetime.


More information:

Dr. Rebecca Rogers Ackermann is with the Department of Archaeology at the University of Cape Town

http://www.uct.ac.za/depts/age 

References for more information:

[1] Nishikimi, M., R. Fukuyama, et al. (1994) "Cloning and chromosomal mapping of the human nonfunctional gene for L-gulono-gamma-lactone oxidase, the enzyme for L-ascorbic acid biosynthesis missing in man." Journal of Biological Chemistry 269: 13685-13688.

[2] Ohta, Y. and Nishikimi, M. (1999) "Random nucleotide substitutions in primate nonfunctional gene for L-gulano-gamma-lactone oxidiase, the missing enzyme in L-ascorbind acid biosynthesis." Biochimica et Biophysica Acta 1472: 408-411.

[3] Swallow DM (2003). "Genetics of lactase persistence and lactose intolerance." Annual Review Genetics 37: 197-219.

[4] Lee-Thorp JA and RR Ackermann (2002) "Lactose intolerance is normal!" Science in Africa, June edition. http://www.scienceinafrica.co.za/2002/june/lactose.htm

[5] Jablonski NG & G Chaplin (2000) "The evolution of human skin coloration." Journal of Human Evolution 39:57-106.

[6] Jablonski NG (2004) "The evolution of human skin and skin color." Annual Review of Anthropology 33:585-623.

[7] Balter M (2005) "Are humans still evolving?" Science 309:234-237.

 

Related websites:

African Origins Month 

Related articles:

"Lactose intolerance is normal!"

 

 

 

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