Be careful where you leave your teeth.

Today we're talking about a tidy little paper published a few weeks ago in PNAS.  Genetic evidence for patrilocal mating behavior among Neandertal groups.   No I didn't know what patrilocal meant, but Wikipedia defines it as when a married couple lives near the husband's family.   The whole paper is open access, so you can read the entire thing if you like.  The authors succinctly describe a group of Neandertal remains found in Spain that consist of closely related males with not so closely related females and a few of their offspring.  The authors conclude lots about Neandertal culture: group size hovers at about 9-12 individuals, brothers stick together and wives come from outside, and spacing of children is about 3 years.  How did they decide all of this?  By sequencing mitochondrial DNA.

The Data
A family of 6 adults, 3 adolescents, 2 juveniles and 1 infant were found at the El Sidron site in Spain.  The authors determined them to be a family group of Neandertals based on the excavation and grouping.  They sequenced, and I'm totally blown away by this, the DNA from the teeth of the skeletons.  These bones are about 49,000 years old, people!  They looked for the Y chromosome to verify and/or determine gender and they also sequenced the mitochondrial DNA (mtDNA).  Not all the specimens had a Y chromosome, either  because they were female or because the DNA amplification didn't work.  But all of them yielded positive results for the mtDNA. They examined a couple different regions of mtDNA, where subtle variations, or polymorphisms, are known to exist (see SNPs).   They found evidence of 3 distinct mtDNA lineages, suggesting the 12 individuals came from 3 separate backgrounds.  The adult males came from the first, as did one adult female.  The other 2 adult females came from separate, distinct lineages.  The adolescents, juveniles and infant were mixed, suggesting they were the offspring of the three females.

What does it mean? 
The group drew several clean conclusions from this small data set.  First, the group size.  12 individuals seemed to be the maximum size for a successful group of this heritage, and this group was reaching the maxima.  There were 3 adolescents in the group, so that might speak to when teenagers were considered grown-up enough to venture out on their own.  The males of the group appeared to be related.  The females were outsiders.  So brothers/male cousins stick with eachother, sisters/female cousins go forth to find their mates.  Finally spacing and family planning.  Because each child bears the mom's mtDNA, it was easily possible to determine which kid came from which mom.  And thus, how often a mom had a baby.  And it appears moms had babies every 3 years or so.  This is totally consistent with other evidence (referenced here) showing the normal pattern of nursing and weaning leads moms to regain fertility about 3 years after each birth.

Kind of a lot to learn from one experiment, no?  I'm blown away by a couple things.  First, there's enough intact DNA in these really, really, reallllllly old bones to get good sequence off of it.  Trust me, I've failed at sequencing gobs of purified fresh DNA, so this little technical feat is not lost on me.  This was actually a huge story last year: the entire genome of Neandertals was sequenced (referenced here - an open access paper so have a gander at it).  Pretty friggin cool.  Second, there exists a mtDB - a database of human mitochondrial DNA sequences.  You can look up any of the 1865 human mtDNA sequences completed and banked there, the polymorphic sites.  It might look like a bunch of gobbledy gook, but there's enough information there for anyone with a buccal swab, a PCR machine, and a couple of primers to determine where they fit in the whole mtDNA spectrum. If anyone wants to pitch in, I'm willing to start my own service.