Around 2.9 million years ago, an ancient hominin in East Africa butchered a hippopotamus and feasted on its pulverised flesh.
Leftovers from the hominin's meaty meal were recently discovered at a site on the shores of Lake Victoria in Kenya by an international team of archaeologists.
Along with the hacked hippo bones were some of the oldest stone tools ever found, reports the team in the journal Science.
"It's the earliest evidence of large animal butchery," said study co-author Julien Louys, deputy director of Griffith University's Australian Research Centre for Human Evolution.
Just who the butchers were is a mystery, but two large molar teeth of a hominin species were also uncovered at the Nyayanga site.
The teeth belong to Paranthropus boisei or "Nutcracker Man", a species on a side branch of our family tree.
The discovery of the teeth, the first of this species to be found in this area, along with the bones and artefacts adds weight to the theory that hominins in our lineage, Homo, may not have been the first to wield crafted tools or eat meat, said study co-author Andy Herries of La Trobe University.
"Generally stone tools and meat eating have been intrinsically linked to the idea of the beginnings of our genus and bigger brains," Professor Herries said.
"This suggests that perhaps there were other hominins on the landscape utilising this material and living in ways that were similar to our direct ancestors."
Uncovering a butchery site
Archaeologists, led by Thomas Plummer of Queens College City University New York, began excavating at Nyayanga on the Homa Peninsula in 2015 after hearing stories of fossil bones and stone tools eroding out of the hillsides.
Today the area is dotted with farms, but 3 million years ago it would have been a grassy wetland.
Working alongside locals, the archaeologists uncovered more than 1,000 fossil remains of elephants, antelope, monkeys, giant crocodiles, extinct species of horses, and hippopotamus.
Lying in the sediment with the animal fossils were more than 300 stone hammers, cores and flakes.
Dating of the site using a number of methods, such as decay of radio isotopes and reversal of Earth's magnetic poles over time, indicated the artefacts were between 3 million and 2.6 million years old, but closer to the 2.9 million mark.
While older stone tools, known as Lomekian, have previously been discovered in Kenya, the tools at this site were fashioned in the more sophisticated Oldowan style.
Instead of simply smashing one rock onto another to break it into smaller, sharper pieces, the ancient tool-maker held a hammer stone to chip away at a smaller rock they rotated in their other hand.
"This gives it much more precision and gives you much more control over the type of stone tool that you're going to be producing," Dr Louys said.
'They're cutting through animal flesh'
Until now, the earliest instances of this style of tool making had been found 1,300 km further north in 2.6-million-year-old deposits at Ledi-Geraru, Ethiopia.
It was unclear who the tool-maker at Ledi-Geraru was, or what the tools were used for, Professor Herries said.
"It's very rare at this time period to actually find stone tools as well as cut mark bone and a hominid.
"So in this case, [at Nyayanga] we've got all three things."
Cut marks on the bones of hippopotamus and antelope, as well as the shape of the edges of tools, reveal the ancient hominin used them to process plants, meat and even bone marrow.
Dr Louys said there was no evidence the hominins were hunting animals, but they were definitely using their carcasses.
"We know from the stone tools that they're cutting through animal flesh."
The question, he said, was how did they manage to hack up a large lump of hippo without becoming prey themselves to large carnivores such as lions, hyenas and giant crocodiles?
"The other interesting thing is the data that we have from this site is before the first evidence we have for the use of fire."
That means they would have eaten the flesh raw or pounded it into a fine mush, a la hippo tartare.
Who was 'Nutcracker Man'?
Paranthropus boisei was a small-brained hominin, closely related to the ape-like Australopithecus known as Lucy (and still referred to by some as "robust australopithecines").
As its nickname "Nutcracker Man" suggests, it had a powerful jaw and large teeth. But it didn't eat nuts.
Analysis of the teeth found at Nyayanga revealed the hominin's diet was heavy in tough grassy material.
It's not the first time the species has been associated with Oldowan tools.
The first fossil remains of Paranthropus boisei — a tooth and partial skull — and the distinctive tools were both discovered years apart in Oldovai Gorge in Tanzania by Louis and Mary Leakey.
But, Professor Herries said, the tools were later attributed to an ancient human in our lineage known as Homo habilis or "Handy Man".
"When they found Homo habilis stuff in the 1960s they suddenly said, 'Oh no, Handy Man must have made the tools,' and Paranthropus got shoved to the side forever," he said.
The earliest fossil of Homo habilis, thought to be the first ancient human to wield tools, found in Ethiopia is around 2.8 million years old.
While we can not definitively say the butchers at Nyayanga were Paranthropus, we now know they were in the area at the time.
"It is possible, of course, that there were other hominins roaming around that were responsible for this, but we don't have any evidence of them in this time period," Professor Herries said, adding that the fossil record for East Africa is very poor.
Filling in the gaps
Benjamin Schoville of the University of Queensland said the discovery combined many levels of evidence about aspects of human evolution that had only been hinted at before.
"It gives a really clear snapshot of what was happening between 3 and 2.6 million years ago," said Dr Schoville, who studies tool technologies at other sites in Africa.
He said the discovery suggested that Paranthropus were very much equal to their early Homo cousins at the time.
But, he said, the big teeth that may have given Nutcracker Man their success may have also been their downfall once more sophisticated tools and, eventually, the use of fire came along.
"That's where our genus, early Homo, picked up the torch and carried it on, becoming us.
"Yet this discovery expands the diversity of behaviours associated with Paranthropus and the complexity of what was even happening in East Africa at this time."
Archaeologists have been working in East Africa since the 1930s, but there is still much more to learn, Dr Louys said.
"There are still so many discoveries to be made."
