Anthropology.net

That is what University of Liverpool’s Emma Nelson probably would have said if she were to meet our hominan ancestors in person. Known to hold true in anthropoids (humans, apes and monkeys), the index (second digit) to ring (fourth digit) fingers ratio or 2D:4D is an indication of how much an individual were exposed to androgen (such as testosterone) in the womb. The more androgen you are exposed to, the longer the ring fingers are (and the shorter the index fingers are compared to the index fingers).

Nelson et al. believe that a high ratio (longer index finger, shorter ring finger) suggests monogamy (or pair-bonded) while a low ratio (shorter index finger, longer ring finger) suggests polygamy (or non pair-bonded). Simply put, individuals with high androgen level is likely to be non pair-bonded and the telltale sign is in the index and ring fingers. Also, some controversial studies had suggested that both men and women who receive high levels of androgen in the womb are more likely to be stronger, faster, and more sexually competitive.

Nelson and her researchers recently looked at the fossils of two Neandertals and one Australopithecus afarensis with complete index and ring fingers to determine their 2D:4D. They found that Neandertals had long ring fingers, suggesting that they were polygamous just like modern day primates that live in groups. A. afarensis on the other hand, had long index fingers. Nelson is puzzled by this discovery. “These were small creatures that probably lived in groups and were being eaten by predators. How do you keep from mating with different members of the group?”, she said.

Indeed it does not make sense for A. afarensis to be monogamous if they live in groups. Notice that Nelson et al. only used one A. afarensis fossil to get the 2D:4D. Perhaps it is not their fault that only one A. afarensis specimen had complete index and ring fingers but such are the dilemma of using fossil specimens to generalize a whole species.The result might just be a statistical outlier. However, I can’t speculate the result or the implications but anyone that are familiar with statistical data knows that a small sample size leads to a higher sampling error. Also what would a 2D:4D = 1 (same index and ring finger length) be?

Interesting enough, John Hawks at John Hawk’s Weblog mentioned the correlation of 2D:4D with male homosexuality (Robinson and Manning, 2000). I would know about this. In fact, my 2D:4D is indeed low.  Robinson and Manning predicted right! Maybe …

I do find both Nelson et al. and Robinson and Manning (2000) research interesting but I would like to stress that the results are mere predictors and correlations. Take it with a grain of salt. Don’t go measuring 2D:4D of your future spouse, boyfriend or girlfriend and accuse them of not being monogamous or a homosexual.

Emma Nelson and her team presented their research at this year’s Society for Vertebrate Paleontology meeting held in Bristol, United Kingdom. Read more about Emma Nelson’s research.

References:

Reilly M. 2009. Human Ancestors Conflicted on Monogamy. Discovery News. Retrieved September 25, 2009, from http://dsc.discovery.com/news/2009/09/24/human-monogamy.html

Robinson SJ. Manning JT. 2000. The ratio of 2nd to 4th digit length and male homosexuality. Evolution and Human Behavior 21(5): 333-345. [doi:10.1016/S1090-5138(00)00052-0]

Originally posted on The Prancing Papio

Human mating systems are mediated by many different variables, from cultural preferences to religious alignments. In some places, people marry and mate within their class or ethnic group. But there are more subtle, almost subliminal conditions behind mating that often affect the outcome of genetic variation and evolution. While I was out of the country, a paper was published in PLoS Genetics which outlined one of these conditions — mate choice and the major histocompatibility complex.

If you haven’t taken an immunology or molecular biology course, you may not know what the major histocompatibility complex (MHC from now on) is. The MHC is the most gene dense region of the mammalian genome and encodes for a lot of proteins act which act as signposts on the cell. These proteins are a crucial part of the autoimmune system. These signposts identify cells as self or foreign to the immune system, ultimately making MHC’s a critical part of disease-resistance. As Dienekes summarized, the evolutionary implications are pretty profound,

“couples dissimilar in [MHC genotype] will produce heterozygous children that will be more capable of fighting disease.”

The three authors of, “Is Mate Choice in Humans MHC-Dependent?,” asked whether or not couples differ more or less in the MHC region of the genome they do across the entire genome? Their testing employed genome-wide genotype data and HLA types in a sample of Yoruban and a sample of European American couples. This allowed them to distinguish MHC-specific effects from genome-wide effects. The group examined whether husband-wife couples were more MHC-similar or MHC-dissimilar in comparison to random pairs of individuals.

Surprisingly, the African couples were not more MHC-similar nor MHC-dissimilar. But across the genome, they were more similar than random couples. How could this be? This could possibly be due to social factors, i.e. mating with genetically close individuals that are within social units, rather than with individuals from the entire population.

European American couples, however, were predominantly MHC-dissimilar. They were drastically so in comparison to the genome, supporting the hypothesis that the MHC influences mate choice in this population. Does this mean that there’s been some sort of evolutionary pressure for Europeans to have more heterozygosity at MHC loci? I’d venture to say yes, epidemics like the Black Plague severely affected European populations but that’s not to say African’s have been disease free. As you may know malaria has constantly been a problem. More studies need to be conducted in other African populations with different mating patterns to fully consider if biological traits like MHC genotype play a significant role beside social traits in the process of mating.

Raphaëlle Chaix, Chen Cao, Peter Donnelly, Molly Przeworski (2008). Is Mate Choice in Humans MHC-Dependent? PLoS Genetics, 4 (9) DOI: 10.1371/journal.pgen.1000184