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ResearchBlogging.orgFor those of you who’ve been following this blog you should be able to recall that last week I wrote about a paper that purported to identify the development of monogamy in the human family tree. For those of you who haven’t been following I still wrote that post. And you can still read it.

The final conclusion of this research was that there was less male/male competition in the hominin lineage pretty much as soon as we diverged from apes. However they couldn’t really identify when we went from not fighting over women to only having one woman in our lives. For all we know early hominins could’ve had permanent relationships with multiple individuals.

Today I report on another paper on this subject that seems to have found a way of rectifying this gap in our knowledge.

At least on paper.

The principle behind this research is not dissimilar to that old nugget of “pop” science regarding finger length and a penchant for homosexuality. Conventional wisdom has it that having a longer fourth digit (from the thumb) than second digit is correlated with being gay (and a longer second digit is correlated with homosexuality in women).

Whilst I neither know nor car enough to tell you whether this bit of banality is true I can say that the 2D:4D digit ratio seems to be correlated with a variety of behavioural traits. This is because it seems to reflect the amount of “prenatal androgen effects” an individual is subjected to in the womb, which in turn influences the development of these traits.

Of all the various things the ratio is an indicator of the most important seems to be correlated with monogamy. Pair-bonded monogamous species like us have a higher 2D:4D ration than more…liberal species such as chimps.

Liberal species...hipppy chimps...see what I did there?

So they grabbed some fossil phalanges and tried to work out what style of relationship our ancestors engaged in. Taking samples from a range of species, including non-hominins (i.e. not on our family tree since we diverged from chimps) they looked at Pierolapithecus catalaunicus, Hispanopithecus laietanus, Ardipithecus ramidus, Australopithecus afarensis, Homo neanderthalensis and even a bit of early Homo sapiens

They then worked out what ratios correlated to what social habits by looking at the finger lengths of 5 living species with a range of behaviours and then compared them to their extinct relatives.

The results?

The two ape species considered were non-pair-bonded polygynous species like most modern apes as was Ardipithecus ramidus. Australopithecus afarensis on the other hand seems to have been pair-bonded like a modern human whilst modern humans…weren’t! Or at least, the early members of Homo sapiens studied. They were more consistent with the digit ratios obtained from modern polygynous groups of people(specifically a tribe of Zulu) , as were the neanderthals.

Whereabouts the various species fall on the 2D:4D scale and which species (and behaviour) they match best with.

This would seem to contradict the research I posted last week and this paper makes no bones about the fact it does fly in the face of canine data

Firstly, canine dimorphism is reduced in hominids compared with other primates. Secondly, estimates of skeletal size dimorphism from small and often spatially and temporally dispersed fragmentary fossils are prone to error.

Firstly, I’m not sure if just listing off a conclusion of investigations into canine size counts as criticism. Secondly, I feel obliged to note that some of the fingers this study used were disarticulated and so the same error could be made here. That said most of the fingers they used were found in situ (in place) and so can be trusted.

So based off that one would be tempted to side with this information over the canine data. However, I’m still unsure of the reliability of these calculations given the link between monogamy and finger lengths was established based on an examination of only 5 species, of which only 1 were pure pair-bonded (humans are weird, not fitting into traditional pair-bonded definitions as we live in multiple male, multiple female societies).

Further the canine data can only really comment on the amount of male/male competition, not the amount of monogamy in the species. If one looks at it like that then there isn’t really much a conflict: Humans remained polygynous throughout their evolution, they just didn’t fight each other for women.

However people seem to be trying to make these two data sets at odds when as far as I can gather they’re simply measuring different attributes. So provided one doesn’t try to over extend the canine data and suggest humans were monogamous due to limited male/male competition we should be fine and another piece of the hominin puzzle can be put in place.

Nelson E, Rolian C, Cashmore L, & Shultz S (2011). Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus. Proceedings. Biological sciences / The Royal Society, 278 (1711), 1556-63 PMID: 21047863

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Jack Aidley · 6th March 2012 at 11:21 am

Is it really valid to consider canine size in the line leading to humans; isn’t part of the evolutionary history of our line a move away from the use of natural weapons to tools? Would humans really have retained large canines for male-male conflict while bringing down mammoths and deer with spears? That would seem most strange to me.

I would argue that canine size is not a useful measure of human mating patterns once we’ve become truly bipedal.

    Adam Benton · 6th March 2012 at 2:55 pm

    But the paper on canine size was arguing even as early as Ardipithecus there was a reduction in sexual dimorphism, which is considerably earlier than the advent of technology (or at least, technology more complex than chimps). So whilst other variables may well influence canine size I don’t think they would be a particularly strong effect that early.

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