Milk through the millennia
Sixty-six million years ago, primitive animals with mammary glands managed to ride out a mass extinction event, with the eventual result that today humans have to pay bills and ponder our own mortality. However, we are the only mammals that can digest lactose into adulthood and blunt these pains with industrial quantities of cheddar cheese, so the verdict is out on whether this situation is a net positive or negative.
But for nearly two-thirds of adults worldwide who are lactose intolerant, milk, cheeses and other dairy products cause a different kind of pain. In the absence of lactase, the enzyme that cleaves lactose into glucose and galactose once the sugar enters our small intestines, lactose passes through whole and becomes food for the microbes that live in our large intestines. Their byproducts then cause bloating, nausea, stomach pains and diarrhea. Moreover, digesting the milk protein casein in adulthood releases insulin growth factor-1, a hormone that can, in excess, Production of lactase also can fall off suddenly in adulthood, although the role that diet — say, eating a brick of feta cheese every day for lunch for several months — may play in this is unclear.
The persistence of lactase in of adults is due to single nucleotide polymorphisms in the gene that codes for the enzyme. Several thousand years ago, the ability to continue digesting lactose was such a strong evolutionary advantage in pastoralist societies that alleles for it independently developed in human populations in both Northern Europe and Eastern Africa, with — in what are today Tanzania, Kenya and Sudan — developing three distinct polymorphisms.
Genetic analyses of modern pastoralists have helped revise our understanding of where lactase persistence developed, but they give a broad range— 6,800 to 2,700 years before the present day — for when those traits emerged.
By analyzing the lipid residues on 125 fragments of ceramic pots recovered from sites in Kenya and Tanzania, scientists at the University of Bristol, the University of Florida and the University of Dar es Salaam have uncovered cultural contexts behind the genetic phenomenon that date its emergence in East Africa to approximately 5,000 years ago.
“Advances in and processing have really helped us pinpoint when we think selection for these alleles appeared and evolved. We now know from the genetic record that this probably happened during a key time period known as the Pastoral Neolithic, which roughly dates to between 5,000 and 2,000 years ago in East Africa,” said , an anthropologist at the University of Florida. “But what we didn’t understand previously is the context in which selection for those alleles actually happened.”
Atomizing pottery
That context is provided by gas chromatography–mass spectrometry. Chromatography first was used to analyze residues in archaeological samples in when French scientists used gas chromatography to detect trace amounts of fatty acids — byproducts of oil — in fragments of jars used to transport goods in the ancient Mediterranean. Their work appeared in the journal .
Today, , an analytical chemist at the University of Bristol uses GC–MS to examine the lipids embedded in the matrices of ancient shards of pottery.
“If you think about an unglazed ceramic pot, and if you had to put some milk in it, or even meat, and just boil it up, what you would actually see is fat globules floating on the top,” Dunne said. “Those globules of fat, lipids, absorb into the unglazed ceramic matrix during cooking.”
Unlike proteins, which are too large to fit in ceramic matrices, lipids are small, and can survive in the matrices of ceramic pots for more than 10,000 years.
“They sit very nicely in the pot until people like me come along and grind up broken potsherds, which are generally only what survives anyway,” Dunne said. “I’ve analyzed pottery from Europe and Africa which is nearly 10,000 years old, and there is actually pottery in Japan that goes back more like 15,000 to 17,000 years and has yielded lipids.”
In , the leader of Dunne’s research group, — who developed the field of organic residue analysis — helped determine that the earliest date that pastoral societies in Anatolia, now Turkey, began consuming processed dairy was 9,000 years ago. This predates the 5,000 year-old pottery that she and Grillo recently analyzed.
“If milk is being processed in the pots, we know humans are consuming milk,” Dunne said. “And once you start processing it and reduce the lactose content, then it makes it much easier for humans to digest. So that’s happening in the Near East around about 7000 B.C.”
From serendipity to eureka
in Nairobi, Kenya, studi