When humans are nursing, we all have an enzyme, lactase, that allows us to break down the milk sugar lactose. However, in our early ancestors, the activity of lactase eventually decreased or stopped entirely. Those modern humans that retain this trait are lactose intolerant as adults. However, as we know, some people are able to safely consume milk (and thus lactose) into adulthood. Sarah Tishkoff and her colleagues summarized their recent findings on lactase persistence a couple weeks ago at the American Association for the Advancement of Science meetings in D.C. From one of Tishkoff et al.’s papers on the subject (see reference below):
These individuals have the ‘lactase persistence’ trait. The frequency of lactase persistence is high in northern European populations (>90% in Swedes and Danes), decreases in frequency across southern Europe and the Middle East (~50% in Spanish, French and pastoralist Arab populations) and is low in non-pastoralist Asian and African populations (~1% in Chinese, ~5%–20% in West African agriculturalists). Notably, lactase persistence is common in pastoralist populations from Africa (~90% in Tutsi, ~50% in Fulani)
What do all these populations with high frequencies of the lactase persistence trait have in common? You guessed it….they all have a long history of cattle domestication. What’s cool about this new study is they show that the genetic mutation that gave rise to lactase persistence in modern Europeans is different from that of modern Africans. So, basically, this trait evolved independently at least twice. It also appears as if the evolution and spread of lactase persistence is consistent with a selective sweep (see this post for more info) that began about 7,000 years ago. So, in other words, it’s spread really fast, which means that it conferred a pretty big advantage to those individuals that had it. For more info see this podcast from Scientific American.
Participate in our poll below…are you lactose intolerant? Can you trace your ancestry back to populations that practiced cattle domestication?
When a mutation arises that confers some sort of advantage, those individuals with the mutation have more kids than those without it. Over time, of course, the mutated gene will become more prevalent in a population (this is simply natural selection). In some cases, other pieces of DNA will hitch-hike along with the advantageous mutant gene because they are linked (i.e., close-by) on the same chromosome and will thus also increase in frequency. A SELECTIVE SWEEP occurs when the positively selected gene and all its neighbors (called a haplotype) become the only variant in a population. So, the result of a selective sweep is a reduction in overall genetic diversity in that region of the genome.
Selective sweeps have certainly occurred in recent human evolution: for example, the genes (and associated DNA neighbors) for skin pigmentation and lactose tolerance appear to have arisen among modern human populations in a manner consistent with a selective sweep.
According to a newly published study in Science, selective sweeps were considered to be a relatively common occurrence among humans. However, the new research suggests that this is not so. From a summary in ScienceNews:
Scientists have favored a model of evolution in which beneficial gene mutations quickly and dramatically sweep through a population due to the evolutionary advantages they confer. Such mutations would become nearly universal in a population. But this selective sweep model may not be accurate for humans, a new study indicates. Human evolution likely followed a more subtle and complicated path, say population geneticists Molly Przeworski of the University of Chicago and Guy Sella of Hebrew University of Jerusalem and colleagues.
It may have been difficult for selective sweeps to take hold in humans because of demographics…[p]eople are scattered throughout the globe, so a beneficial mutation would have a long way to spread. Such a mutation would have to have dramatic effects on evolutionary fitness to go global.
See our keynote speaker Dr. John Hawks talking about rapid genetic evolution among modern humans (“Rapid evolution: Can mutations explore historic events?”) at the Council for the Advancement of Science Writing.
Well, according to Ernst Mayr’s Biological Species Concept, which holds that species are “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups,” the newest genetic data (see a previous post on this blog) suggest that perhaps they should be considered the same species. A nice summary of the debate is provided by Ann Gibbons in Science. While some paleoanthropologists (including our keynote speaker John Hawks, who is quoted in the piece; check out his blog post on the subject) consider Neandertals and modern humans to be the same species, others maintain that the two are distinct species because the anatomical, developmental, and behavioral differences between Neandertals and modern humans are much greater than what we see among any modern population.
Check out Gibbons’s piece and tell us what you think. Are we the same species or not (remember, it also depends on what species definition you decide to use)?
It is likely that Dr. Hawks will be addressing some of these issues in his keynote lecture (we now have a title for the talk: “Neandertime: Deciphering the Secrets of Ancient Genomes.”)
This is only tangentially related to our theme, but interesting nonetheless…
Orangutans are separated into two populations (Sumatran and Bornean) and, according to many researchers, are distinct species. A recent study in Nature provides us with a draft genome from a single Sumatran individual and shorter sequences from a handful of Sumatran and Bornean individuals. What is really interesting here is that the Sumatran/Bornean speciation time is estimated to be about 400,000 years ago, which is much, much more recent than the estimate provided by mitochondrial DNA from a previous study (about 3.5 million years ago). As the authors of the new study point out, this “underscores the complexity of the orang-utan speciation process.”
We run into the same sort of issues when estimating divergence times for humans and chimps and when we compare the genomes of modern humans. It really does depend on what part of the genome you are looking at.
Just as we are trying to digest the implications of the draft Neandertal genome (which, by the way, suggests that Neandertals contributed up to 4% of their genomes to non-African modern human populations), a new study published in Nature by David Reich (Harvard Med School) and colleagues reports the genome of an unclassified (all we have is a pinky bone and an isolated tooth) ca. 40,000-year-old hominin from Denisova Cave in southern Siberia. The genome appears distinct both from that of European Neandertals and contemporary modern humans. However, there is evidence that early modern human populations interbred with these “Denisovans” and, in fact, modern Melanesian populations (represented in the paper by genomes from Papua New Guinea and Bougainville) appear to have received approximately 4% of their genomes from this extinct group of humans.
So, taken together, these genetic data seem to indicate that modern Melanesians derive up to 8% of their genomes (4% Neandertal and 4% “Denisovan”) from now-extinct human groups. Pretty cool stuff.
Check out the summaries from Science News and Nature for more information, and our keynote speaker John Hawks’s weblog provides very detailed commentary on these exciting findings.
UPDATE 12.23.11. Dr. Hawks is also interviewed in an NPR story from Dec. 23 that summarizes the implications of these data.