Several close relatives of our species, Homo sapiens, have walked this Earth since the genus Homo evolved more than 2 million years ago. These hominins lived in diverse habitats and challenging environments. Some even crossed paths and interbred.
Scholars have much debated our current exclusivity. Some have proposed that H. sapiens' better technological abilities may have given us an advantage over the rest. Others have suggested we may have eaten a more varied diet or were more efficient runners than other hominins.
Meanwhile, other researchers posit that, given high levels of interbreeding, perhaps some hominins did not go extinct as much as merge completely with our gene pool.
Researchers have also hypothesized that climate change could have played a role in the extinction of Homo species. In a new study, published in the journal One Earth, a multidisciplinary team of scientists from Italy, the United Kingdom, and Brazil make the case that this factor was the major driver in the extinction of other hominins.
The authors believe the findings could serve as a warning as humanity faces human-made climate change today.
"Even the brain powerhouse in the animal kingdom, [the Homo genus], cannot survive climate change when it gets too extreme," says paleontologist Pasquale Raia, of the University of Naples Federico II, one of the study's authors. "People should mind that, given the current mayhem we are causing."
For this study, the team focused on just six of the recognized Homo species: H. habilis, H. ergaster, H. erectus, H. heidelbergensis, H. neanderthalensis, and H. sapiens. They omitted others because the available fossil records were too limited for their analysis.
Using a fossil database spanning 2,754 archaeological records, the researchers mapped out where these species lived over time - linking both fossil evidence and tools associated with each species to various locations and time periods.
They also applied a statistical modeling technique called a past climate emulator that uses available records to reconstruct climate conditions, including temperature and rainfall, over the last 5 million years.
"This offers a picture of the tremendous effects that climate adversities have had," says anthropologist Giorgio Manzi.
For three of the five extinct species - H. erectus, H. heidelbergensis, and H. neanderthalensis - a sudden, strong change in climate occurred on the planet just before these species died out. Climes became colder for all three, drier for H. heildelbergensis and Neanderthals, and wetter for H. erectus. According to Raia, the change in temperature was roughly 4 to 5 degrees Celsius, on yearly averages.
The researchers further assessed just how vulnerable these species were to extinction by trying to determine their tolerance to climate change over time, using their presence in various locations as a clue to their preferred niche.
The team determined that, before disappearing, H. erectus and H. heidelbergensis lost more than half of their niche to climate change. Neanderthals lost about one-fourth. Food sources likely dwindled as habitats changed, and cold may have threatened survival for species adapted to warmer climes.
This climate explanation does not necessarily mean that other drivers of extinction weren't important too - the authors note that competition with H. sapiens, for example, could have made things worse for Neanderthals - but Raia and his colleagues believe their analysis reveals "the primary factor" in past Homo extinctions.
The extinction of Neanderthals has been studied - and debated - quite a bit, but the loss of other hominin species has had little attention, says archaeologist Tyler Faith, from the University of Utah, who was not involved in the study. This new study represents the first attempt to understand how multiple Homo species died out across large swaths of space and time, he says.
"But I think it's a bit early to discount other potential extinction mechanisms," Faith adds. He notes that the limited fossil record for some species makes it difficult to have a full picture of the environmental or climatic conditions that other Homo species could handle.
Similarly, anthropologist Giorgio Manzi, from Sapienza University of Rome, who was not a contributor to the study, notes that many elements should be taken into account to explain the disappearance of past Homo species.
The relationship between climate change and extinction is complex, he says, and one doesn't always lead to the other: "Various abrupt climatic breakdowns and environmental crises are known during, at least, the last million years. These circumstances did not always lead to extinctions."
Still, Manzi believes the new work makes a reasonable case that climate change can have a big impact.
"This offers a picture of the tremendous effects that climate adversities have had on human populations of different species," Manzi says.
With the planet projected to warm as much as 5 degrees Celsius above preindustrial levels by 2100, more climate challenges lie ahead.