Australian researchers say the discovery of a two million-year-old skull in South Africa sheds more light on human evolution.
The skull was a male Paranthropus robustus, a "cousin species" of Homo erectus - a species thought to be a direct ancestor of modern humans.
Both species lived at the same time, but Paranthropus robustus appeared earlier.
The research team said the discovery is exciting.
"Most of the fossil record is just a single tooth here and there, so having something like that is very rare, very lucky."
The researchers at Melbourne's La Trobe University found the skull fragments in 2018 at the archaeological site Drimolen in northern Johannesburg.
Archaeologists then spent the past few years putting pieces together and analyzing the fossil. Their findings were published in the journal Nature, Ecology and Evolution on Tuesday (10/11).
Co-researcher Jesse Martin told the BBC that handling the fossil pieces is like working with "wet cardboard", adding that he used plastic straws to suck the last traces of dirt them.
It was discovered a few meters a place a similarly aged child skull Homo erectus had been discovered in 2015.
Three species of hominids (human-like creatures) are believed to have lived in South Africa at the same time, competing with each other.
The discovery of the skull would be a rare example of "microevolution" within the human lineage, said Martin.
Paranthropus robustus had large teeth and small brains, unlike Homo erectus, which had large brains and small teeth. It is believed that the former's diet involved eating mainly hard plants, such as tubers and bark.
"Over time, Paranthropus robustus probably evolved to generate and withstand superior forces produced during biting and chewing food that were difficult or mechanically challenging to process with its jaws and teeth," said Leece.
Scientists said it is possible that a wetter environment caused by climate change may have reduced the amount of food available to them.
Meanwhile, Homo erectus, with its smaller teeth, was more likely to eat both plants and meat.
"These two very different species (...) represent divergent evolutionary experiments," said Leece.
"Although we are the lineage that won in the end, two million years ago the fossil record suggests that Paranthropus robustus was much more common than Homo erectus."