Category Archives: sex

Sex and Evolution?

I like Michael Stoddart’s books in general–he has some great contributions to make and is one of a few popular scientists promoting olfaction. While reading his most recent book, I have been a bit frustrated by the simplistic view of human evolution and behavior. A recent Guardian piece by him “Smell evolution and the sex brain: Why we’re monogamous and use perfume” captures the source of my frustration. As a biological anthropologist, I find it hard to read the following sentence:

To keep male and female together to provide protection for babies, a suite of anatomical and physiological features evolved to promote the constant availability of sex throughout the year – irrespective of the monthly ovulation cycle.

The argument is that a suite of traits evolved (e.g., reduced sexual dimorphism, hidden estrus) to render human females receptive to sex at any time and this has led to monogamy–meanwhile male receptivity to sex is used as an explanation for purported male promiscuity. Huh! The constant male bias in science is at the heart of taxonomy–our class is called mammal because male scientists felt the key trait of mammals was the use of mammary glands to feed offspring.

Increasing the diversity of voices in academia has allowed us, slowly, to move away from teleological explanations for human behavior based on western society. In biological and evolutionary anthropology, human reproduction is a hot topic and more complicated. Stoddart does qualify his statement a bit:

Yet Homo Sapiens is the only species among the 5,500 kinds of mammal to maintain monogamous family relationships – or at least serially so – and to live in densely populated areas. This combination is extremely rare in nature.

Marriage, as an institution is barely thousands of years old (our species is 200,000 years old) and the concept of marrying for love younger still. Divorce is higher today partly because there are fewer economic and political structures keeping people together–religion is what is left and that doesn’t appear strong enough for most people–divorce was central in Henry the VIII’s split with the Pope. Most cultures are polygamous even if most end up practicing monogamy (mainly due to financial and/or political constraints–not enough money or power to gain more spouses). Perhaps the clearest statement we can make on pair-bonding is that humans can, and often do, come together in a pair-bond for a period of time with a goal of child rearing but this shared interest isn’t immediately linked to sexual monogamy–they are separate issues. The period of shared interest (if it occurs) enables the child to reach a point where the ‘village’ can take on some of the burden through formal and informal education. But, even western society regularly abandons its children–part of the year, I live next door to a youth shelter and drop-in center so I see it daily.

I suppose most humans are humanists–Jon Marks is a biological anthropologist who has written many books on the subject from an evolutionary perspective with a goal to distinguish us from all the other primates. I am not a humanist even if I do appreciate what we have accomplished as a species (there’s a lot to be ashamed of too…). I think there is an inherent fallacy in not recognizing that we are animals and that we cheat and lie and love and, yes, react to odors just as animals do. We may be enculturated to curb instincts but the instincts that we are enculturated to curb and how we do so vary cross-culturally. The goal to overcome our instincts with reason is a cultural one, not a biological or evolutionary one.

And, contrary to this blanket statement:

Today we have a global fragrance market equal to the GDP of a medium-sized country. But because our nose (unlike the VNO) ultimately sends all smells for rational analysis by the brain, we do not slavishly respond to sex smells in the way dogs or mice do. An alluring perfume may help a relationship, but no perfume comes with a guarantee!

odors are first processed in the areas of the emotional center of the brain where memories are also deal with–we react to odors before the frontal lobe (where reasoning attempts to modulate instinct) gets the data and formulates a response. Maybe we wear perfume because it smells good–it takes us to places we want to be or reminds of us of memories we love or smells like things we love to eat–maybe wearing perfume is about sensuality not mating. Why is so much academic work reductionist? But, perhaps that is why I am a biological anthropologist, rather than a biologist. Still, I take the point that we may not react to odors with the full behavioral response other animals might, but we react nonetheless. The closing statement of the piece is perhaps the strangest, and to an anthropologist, the most off-putting:

And so we can live in at least relative harmony with our fellows, benefitting from the long-term genetic and evolutionary advantages provided by monogamy, while participating socially in everything society has to offer.

There probably should be more biological anthropologists writing popular press books on human evolution and this gives me even more motivation to get my long overdue book Smell of Evolution out!

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Filed under anthropology, Anthropology and Evolution, Olfaction, primate social life, religion, Science, Senses, sex

The Scent of a Man

A new study (published here) suggest that scientists unable to replicate  behavioral studies in rats and mice may be due to the presence of male researchers.

The presence of male experimenters produced a stress response in mice and rats equivalent to that caused by restraining the rodents for 15 minutes in a tube or forcing them to swim for three minutes. This stress-induced reaction made mice and rats of both sexes less sensitive to pain.

The chemical signals emitted by males of any species are detectable by other species. Since males secrete these pheromones at higher concentrations than women, the effects tend to be limited to male researchers. Rats and mice acclimatize over time to the male researchers, suggesting an ‘easing’ in period prior to experimentation or perhaps, even better, even more effort to promote women in science!

Since there is growing evidence that humans respond to pheromones, I wonder if there is a similar effect caused by male researchers on human subjects; namely, is stress induced in males and females when experiments are conducted by men? Outside the lab, does the scent of a man induce stress and reduce pain response, but in a good way? I’ll end with ‘Boys‘ by Robots in Disguise.

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Clandestine trysts and human evolution

Recent advances in the field of paleogenomics (the study of ancient genomes) have uncovered the story of inter-species mating in those early days out of Africa before dispersal into Eurasia. Prior to these studies we’ve had little evidence supporting either cultural interaction with archaic humans or inter-breeding.

Clandestine trysts or common practice? The draft sequence of the Neanderthal genome published in 2010 revealed that we mated with Neanderthals in the Near East enough to share 1-4% of our DNA with them. On the heels of the draft sequence of the Neandertal genome, the same team published the Denisovan genome using DNA extracted from an exceptionally preserved finger bone from remains found in Denisova Cave in Siberia. The archaeological data at Denisova show a mixed toolkit with elements of Upper- and Middle-Paleolithic industries. Molar morphology indicated Denisovans were distinct from both modern and known archaic humans. The genomic data indicate Denisovans were indeed a new species with unique genomic markers. The power of modern genomics allows us to also find evidence of mating with modern humans, specifically modern Melanesians who share 4-6% of their genomes with Denisovans. Were these matings clandestine trysts or was there more at stake—some flow of genes to modern humans that helped us adapt to the novel environments of Europe and Asia?

Genomic breadcrumbs? Preliminary comparisons between Neandertal and human genes indicate significant differences in aspects of cognition, metabolism, and skeletal and skin morphology. But what about the inherited portion of the genome? Does it have a role in any functional aspect of our biology and physiology? Dr. Green, lead author on the draft sequence of the Neandertal genome described the inherited portion as ‘sparsely distributed across the genome, just a ‘bread crumbs’ clue of what happened in the past’. But two new papers tell a different story, suggesting that inter-breeding may have significantly contributed to successful adaptation to the new environments of Eurasia.

In the summer of 2011, another team identified a Neandertal origin for a unique cluster of co-inherited gene variants (a haplotype) in the non-coding segment of the dystrophin gene on the X-chromosome. This haplotype occurs at a frequency of 9% in all modern non-African human populations and likely first appeared in the genome prior to or very early in the migration out of Africa. The authors of the study posit that either this genetic (and/or cultural) exchange enabled successful modern human adaptations to the novel environments of Eurasia. There is an intriguing possibility that human males left Africa in greater numbers and mated with archaic females: an earlier study on modern human variation found the X-chromosome experienced more than expected genetic drift at the time of human migration out of Africa, a pattern not found in the migrations into East Asia and Europe.The authors of this study conclude that female effective population size was reduced compared to males due to some sex-biased process or natural selection affecting the X-chromosome in non-African populations.

Speculations that inherited genomic material conferred a fitness advantage gained further ground in a study published this summer in Nature. A Stanford University team identified HLA gene variants that are rare in modern Africans but significantly present in West Asians, again suggesting genetic admixture outside Africa prior to Eurasian expansion. HLA class I genes are critical to the immune system because they target and destroy pathogens. The authors of the study argue that inter-breeding restored HLA diversity that was reduced by a population bottleneck in migration out of Africa, citing examples of similar events in the evolutionary genetic history of the peopling of the Americas. Not only was diversity restored to modern humans but new immune variants specifically adapted to local pathogens may have been acquired in the process as well. HLA-A*11 (associated with Epstein-Barr virus protection), for example, has become a dominant form in non-African populations occurring at rates of 64% in East Asia and Oceania (and even more in Papua New Guinea) suggesting strong selection. The high frequency of these gene variants (as compared with other regions of the genome) may be explained by the need for the immune system to be flexible to new pathogens (particularly rapidly evolving viruses), rendering it more susceptible to the forces of natural selection.

The future of the past. If the data from these various studies is supported by future work, we may end up rethinking our relationship to our closest cousins – were we separate species? Even field biologists who have the array of genes, biology, physiology and behavior sometimes have trouble determining whether two groups be classed as separate species or not. For those working in paleoanthropology or paleogenomics, the dataset is even more limited and the creation of new taxa is temporary pending further data but useful as a heuristic tool.

The field of paleogenomics is relatively young and has a tremendous number of methodological and technical challenges to overcome before we can comfortably say that the sequences yielded are authentic and reliably represent the genetic data. A major challenge is verifying and authenticating the endogenous (or local) DNA in a specimen that has been potentially contaminated by microbes and human researchers. The past decade has been punctuated by marvelous advances that have helped us better understand recent human evolution and ourselves. The possibility that our advantage in global colonization derives from early acts of inter-breeding is a fascinating one. With the rapid advancements in technology and increased interest in ancient DNA, the future looks promising for unraveling the story of the immediate past.

  1. Green, R. E. et al., Science 328, 710 (2010).
  2. Reich, D. et al., Nature 468, 1053 (2010).

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Filed under anthropology, Anthropology and Evolution, Denisovan, Neandertal, race, Science, sex