(Ms Felicia Gomez and Dr. Jibril Hirbo give a talk at the Smithsonian's Human Origins Topic. Photo by the author.)
The human is the only mammal who continues to drink milk throughout adulthood. Most mammals completely loose the ability to digest milk after weaned, a gradual process that starts as early as 16 weeks. Humans can normally digest milk for the first 5-7 years of life, but after that, lactose digestion is slowly lost, and most people become what we know as lactose intolerant. In fact, the majority of the world is lactose intolerant, to various degrees; not surprisingly, considering this is the norm in most mammals. Then why are some humans able to consume milk and drool over ice creams, butter and cheeses?
Lactose intolerance has someone else to blame: lactase!
Lactose is the main sugar present in milk. As a rule of thumb, sugars are carbohydrates whose names end in “ose” (think glucose, fructose, galactose, lactose). The enzyme lactase – where “ase” endings indicate an enzyme name – works as a little molecular engine, breaking down lactose into smaller pieces: glucose and galactose. Lactase is abundant in infant mammals, but much less present in adults. The deficiency of this breaking down enzyme results in lactose malabsorption and is the cause of lactose intolerance.
However, in many population groups where milk is a key dietary source, humans adapted and continue to digest lactose after adulthood. This trait is named “lactase persistence”, and I guess you could say it is the opposite of lactose intolerance.
(Pockets of lactase persistence shown in pie charts. Ingram et al, Hum Genet 124:579–591, 2009)
Lactase persistence is well know in Europeans
Lactase persistence has evolved independently in several population groups around the world. It is assumed the environmental pressures drove adaptation in pastoral communities who rely heavily on milk products for nutrition and even for a water source. In other words, if a population is required to consume milk derivatives, given time its DNA could adapt to process it. And have done so, a few times and in different places of the world.
(Two groups that have independly adapted to lactose digestion: the swiss and the masai. Photo credit: the evolution group at Berkley)
I first got interested in this topic after I participated on the “Human Origins Topic” at the Smithsonian, where Dr. Jibril Hirbo and Felicia Gomez, from the Sarah Tishkoff group, talked about their work (see top photo). They collect DNA samples from many african populations and track down differences in DNA sequences of specific genes. A single DNA nucleotide base change is named SNP, for Single-nucleotide polymorphism (pronounced as “snip”), and Tishkoff’s group has been cataloging SNPs for lactase persistence in african populations.
Lactase persistence in europeans is well known and documented, and is caused by the SNP C/T-13910 (the numbers represent the position in the gene where the DNA change occurs). 90% of northern europe (specially in Sweden and Denmark) is lactase persistence, and ~50% of southern europe (Spain, French).
The cause for lactase persistence in african populations was still unknown, and was studied by Tishkoff, whose findings were published in the “Convergent adaptation of human lactase persistence in Africa and Europe“.
(photo from the evolution group at Berkley)
Capturing SNPs in remote african locations
In order to detect SNPs associated with lactase persistence, Tishkoff’s group collected and sequenced DNA from 470 individuals from 43 african ethnic groups (Tanzanias, Kenyans and Sudanese). A majority of this collection work was done by Dr. Jibril Hirbo, who explained his approach:
“The biggest challenge was to get to communities we intended to sample because of poor infrastructure. Once we got to our destination we usually approached the community leaders and talk to them about our research, then the leaders organize something like a ‘townhall” meeting where we explain our research to the villagers and make sure they understand what we were doing. It was easy talking to the people because I spoke the local language and know the cultures…so I just broke it down to them in the way they could understand.”
The test – named LTT for lactose tolerance test – is very straightforward, as Dr. Hirbo points out:
“The sampling was in two part – initial blood/saliva collections for the study of genetic variation of human population and second lactose tolerance test that involved drinking the orange solution that contain lactose sugar that is found in glass of milk followed by monitoring blood glucose level in blood drops from finger prick over one hour period.”
The DNA collected from the saliva is brought to lab, and sequenced, to identify SNPs correlated to the lactase persistent populations. The correlation between SNP and phenotype (the presence of lactase persistence) can be obtained by matching the DNA with the results from the LTT.
They found that each population group has a different mutation that resulted in lactase persistence. None have the european SNP (C/T-13910), and instead, each of the tested african groups displayed unique SNPs (G/C-14010, T/G-13915 and C/G-13907).
This is a clear case of convergent evolution: where 4 distinct variants correlated with lactase persistence independently arose in the world. These genetic variation was associated with pastoral groups and its migrations. Tishkoff’s group even goes further to say that the pattern of genetic distribution might match the cultural and linguistic distribution in Africa.
(Scheinfeldta et al, PNAS vol. 107 no. Supplement 2 8931-8938, 2010)
Can we safely conclude that the genetic spread accompanies culture? Pastoral, ethnic, and lactase persistent groups appear to indicate that.