by Kristen Minogue
Artist’s depiction of the prehistoric synapsid Varanodon agilis (left) under attack from an ancient amphibian. (Wikimedia Commons/Smokeybjb)
Milk—the white, calcium-rich liquid common to mammals and refrigerators across the globe—may have evolved long before the mammals that secrete it. It may have evolved even before dinosaurs. It’s an idea SERC lactation expert Olav Oftedal proposed a decade ago and is now gaining momentum among biologists who study the evolution of what we drink.
Milk’s history begins just over 300 million years ago, in a time geologists call the Carboniferous Period. Lizard-like vertebrates called amniotes shared the land with the then-dominant amphibians. Unlike amphibians, amniotes laid eggs with parchment-like shells that could survive out of water in moist places. Then, about 310 million years ago, amniotes split. Sauropsids, ancestors of birds and reptiles, evolved hard-shelled eggs, while adults evolved the tough scales that would eventually shield the dinosaurs.
But synapsids, the line leading to mammals, took a different path. According to Oftedal, synapsids continued to lay eggs with parchment-like shells able to absorb moisture. Meanwhile, adults further developed the secretory glands in their skin. By secreting a fluid rich in antimicrobial compounds, parents could keep eggs from drying out and provide disease protection. Over time, these compounds also became a nutrient source–either directly across the eggshell or for young after hatching.
300 million years ago: Synapsids like this Dimetrodon secrete liquid to moisturize their eggs. (Charles R. Knight)
Fast forward 100 million years and several mass extinctions into the future. Mammaliaforms—the nearest ancestors of modern mammals—have emerged. In the shadow of the dinosaurs they’re tiny, generally the size of modern shrews and rats, and warm-blooded. Their eggs must have been small as well and would wither quickly if incubated without liquid from outside. Meanwhile, mammaliaforms are losing teeth. Most animals of the period go through many sets of teeth as their jaws grow. But later mammaliaforms have only two: an early set of “milk teeth” and adult teeth. Young hatchlings, unable to feed on their own, rely on nursing.
225 million years ago: Small mammaliaforms like the Morganucodon nurse their young. (Michael B. H.)
Neither egg incubation nor the decline in tooth sets would have been possible if milk and lactation were not already fully developed. This led Oftedal to trace their origin even earlier. He says it’s unclear which of our ancestors first began secreting fluid to nourish young offspring, and not just as a convenient way to moisturize eggs. But somewhere between synapsids and dinosaurs, the liquid we call milk arose.
Today: Weddell seals, the southernmost lactating animals, produce some of the world’s richest milk. (Samuel Blanc)
“People sort of thought it somehow just appeared, and that it was unique to mammals, and therefore it was a mammalian invention,” Oftedal said. “And I’ve said, basically, no. We inherited it.”
When Oftedal first voiced the idea 10 years ago, paleobiologists did not immediately embrace it. But many biochemists, long puzzled by the mysteries of lactation, were relieved. And the molecular record is catching up. Closer inspections outlined in his latest article, published in November, increasingly suggest milk is far more ancient than mammals.
First and foremost is calcium. The calcium in milk is transported primarily by a group of proteins known as casein proteins. Casein proteins have three major types, and all three appear in every mammal species on Earth. This immediately implies they were inherited from a pre-mammal ancestor. In 2003, a researcher named Kawasaki traced them to an even older protein group. Following Oftedal’s hypothesis, he suggested in 2011 that these ancestral proteins could have been delivering calcium to eggs even before the first synapsids.
Then there’s lactose. Our pre-mammalian ancestors were lactose-intolerant. This poses an evolutionary riddle for scientists, who wonder how lactose secretion could evolve in species whose offspring couldn’t digest it. The answer may lie in the proteins that make lactose—particularly one called alpha-lactalbumin. Scientists now think this protein did not initially produce lactose on its own, but rather made larger, lactose-containing sugars with antimicrobial properties. This protein evolved from lysozyme, an antimicrobial constituent, roughly 310 million years ago, when synapsids emerged.
The new discoveries suggest we are not the only creatures to produce milk, just the only ones to have survived. But it’s not so surprising with hindsight. “The only reason why people didn’t realize this was because there’s nothing else that lactates,” Oftedal said. “None of the other lineages that would have lactated left any descendants.”
Images of Morganucodon and Weddell Seal used via the following Creative Commons license. Dimetrodon image (1897) is in the public domain due to creation before January 1, 1923.
