Skin cancer selected our ancestors?
The mysteries surrounding how humans evolved in Africa are legion. In the savannah habitat thought to have been the "nursery" for early hominin species, the heat and the bright light would have influenced skin particularly. Whether sweat glands or skin colours adapted during the speciation would be interesting to discover.
Cancer risk is thought to have been a minor but important influence on melanin distribution in the epidermis of the skin. The amazing date of this change would be only 1 or 2 million years ago. But this week, a very interesting theory could break new ground in our consideration of how we came about!
Protection from the ultraviolet light of open habitat would be the obvious function for melanin and its gene, melanocortin 1 receptor (MC1R) allele. Cancer incidence among human albinos in 2 locations support a theory that the 3 types of skin cancer could have helped select for darker skin colour in these early human species. In modern humans, cancers affect older individuals, but dark skin reduces cancer risk by several orders of magnitude. There isn't much room for white-skinned blond(e)s when the sun shines harshly.
Basal cell cancer, squamous cell carcinoma and malignant melanoma are the most common skin cancers currently affecting people, often when UVB-damaged cells in early life becoming carcinogenic later. This type of UV affects the nucleotides of chromosomes in these cells in white-skinned individuals. DNA repair is very efficient in many people, otherwise cancer would be prevalent in very young people with white skins. Black skins do get these cancers to a small extent, but usually on the less pigmented soles and palms.
Bacteria, fungi and many invertebrates use melanin in the same way as humans, to prevent chromosomal damage, as well as for display, camouflage, defence signalling (cuttlefish ink) and vision. High altitude organisms and even those living on the walls of Chernobyl exploit melanin to its fullest capacity to absorb both UV and gamma radiation. The type of melanin in white skins however, can actually encourage carcinogenesis. This pheomelanin is more photoreactive, making free radicals that will certainly help the breakdown of nucleotides!
Latitude and altitude affect your exposure to UVB. Your behaviour will obviously influence exposure, for example if you only came out at night and sun-tan will obviously help to produce more of the useful eumelanin instead of the pheomelanin. Unfortunately, red-haired people can produce very little eumelanin. As most of us are probably already aware! This explains the original distribution of skin colours, and their reflectance, around the earth's latitudes, as correlated with satellite measurements of UVR levels.
East Africans migrating for the first time into Europe seem to have evolved the first paler skins, between 50,000 and 80,000 years ago. 5 genes were involved, with sigs of such recent selection very obvious in peoples' genomes. Ale skins can generate vitamin D, as we have known for a long time, and apart from some inner city occurrences, rickets is avoided because of this skin's ability. Would rickets have had much effect on darker skinned individuals migrating to the sunless north? Not if a fish diet was available, as it certainly was. Cold and frostbite affect pale skin less, but so many factors could have influenced these colonists, it would be very difficult simply to guess.
More research is available on how black pigment helped early hominins. For a start, body hair was reduced, apart from the head, where UVR exposure is most obvious. Thermoregulation would be an acceptable reason for this change, along with those sweat glands, which still vary in distribution among modern humans. The nearest relative, the chimpanzee, is very pale-skinned, with the melanin only in hair follicles.
Homo ergaster or Homo erectus would have appeared in the African savannah with black skin around 1.7 mya. Climate change may have swept them onto the open plains, perhaps as hunters of the different fauna. Brain, sweat and black skin would be the 3 most obvious physical changes, if this particular theory is correct.
Albinos in every animal occur regularly and naturally, but suffer from high rates of melanoma when experiments have been conducted. Homozygous albinism in Africa people happens quite often, and in all populations at that rate. This commoner type of mutation can produce pheomelanin, to protect them slightly. Cancer rates are very high (about 1000X higher than normal) in Nigeria, Tanzania and other equatorial regions.
White skins are not alone in suffering here, but most albinos have lesions (precursors to cancer )by the age of 20. These tend to go away, but after this age, cancers begin to appear in almost all albinos on the face and shoulders particularly. In Nigeria, 50% of albinos were diagnosed with cancer by the age of 26, and Tanzania had similar figures, though given as 50% at 20 and 80% at 30 years old.
You can't expect a peasant farmer to remain indoors, as white people would. Skin cancer therefore is the major cause of death before African albinos leave their 30s. Reproductive life for an ancient hominin race, if it were subjected to the same conditions would be much reduced, and therefore a selective advantage would be huge for any skin darker than an ancestral white skin. To leave the forest and hunt on the plains posed a long-term adaptive problem. How to survive in the sun?
Lifestyle, occupation, awareness, behavioural modification and the use of clothing would all have affected these giant steps for hominin-kind. Young males hunting in the open would not survive to reproduce, given any comparison with other animal species. Successful Darwinian survival of the fittest would involve those with more melanin if this theory is tenable. Africans have very stable MC1R variants, because this blackness is virtually printed on their genotype. No population has survived there for a million years without some variant of this gene (allele.) It is critical in all of our evolutions, as are those first great black colonists of the plain and, later, the north.
We must congratulate Professor Mel Greaves of the UK Institute for Cancer Research for his convincing and truly intellectual approach. He has published his paper today in the Proceedings of the Royal Society B.