Two years ago, a team of researchers excavated the earliest and best-preserved specimen of Paranthropus robustus—our ancient cousins with jutting cheekbones, large teeth and small brains that lived from 1.2 to 2 million years ago—on an archaeological dig in South Africa's Cradle of Humankind. With it, the team also unearthed the most compelling evidence that species in the human family tree were able to evolve in response to quick changes in the climate, reports Nicholas St. Fleur for the New York Times.
The two-million-year-old skull, labeled DNH 155, belonged to a male from the Drimolen cave system. Compared to other P. robustus males recovered from a nearby cave system called Swartkrans, DNH 155 was much smaller and had more female-like characteristics, according to a study published Monday in the journal Nature Ecology & Evolution.
Archaeologists had originally believed that for the most part, female P. robustus were smaller, less burly and lived in Drimolen; males, which were heavier built, lived in Swartkrans, reports Peter Dockrill for Science Alert.
“Now, that didn’t seem right to me,” Angeline Leece, a paleoanthropologist at La Trobe University in Melbourne, Australia, tells the Times. “What it looked like to me instead is that we have males and females in Drimolen, and males and females at Swartkrans, but the Drimolen ones were just overall smaller.”
But there was a 200,000-year difference between when P. robustus occupied the Drimolen caves and then Swartkrans. Around that time—about two million years ago—the region underwent a dramatic change in climate. The lush, green grasslands transformed into drier and cooler fields, altering the ecosystem's vegetation and the plants available for P. robustus to feast on, a press release says.
The plants became tougher and harder to chew, so DNH 155 and other P. robustus wouldn't have been able to munch on them. As time went on, natural selection favored individuals with stronger chewing muscles and tougher teeth, so they passed those traits down to their offspring. Then, some 200,00 years' worth of evolution later, P. robustus ended up with sturdier jaws.
The variation between the P. robustus specimens found in Drimolen vs. Swartkrans wasn’t due to physical differences between males and females, as originally thought, Jesse Martin, a doctoral student on the project, tells the Times. Instead, it was a result of environmental pressures that forced the species to evolve "massive chewing and grinding [machines]" to survive.
"The Drimolen fossils represent the earliest known, very first step in the long evolutionary story of Paranthropus robustus," Martin says in another press release.
This find is the first evidence of microevolution—the changes within a population of one species over time—in early hominids, reports the Australian Associated Press.
"Like all other creatures on earth, our ancestors adapted and evolved in accordance with the landscape and environment around them," Andy Herries, a paleoanthropologist at La Trobe University, says in the press release. "For the first time in South Africa, we have the dating resolution and morphological evidence that allows us to see such changes in an ancient hominin lineage through a short window of time."
In addition to helping archaeologists piece together the evolution of our ancient ancestors, this discovery also serves as a warning for other scientists to not immediately jump to the conclusion that every odd-looking specimen is a new species. Instead, the specimen could be from a known species, just in a different time or place.