The story of human evolution is a weaving, complex web that involves a number of different species known from the fossil record. Some of these species are considered direct ancestors of modern humans, while others are considered offshoots that share a common ancestry with modern humans but which ultimately proved to be evolutionary dead ends.

One of the central characters in this evolutionary tale is Homo erectus, the first species of the genus to migrate out of Africa and spread across Eurasia, as well as the first human known to develop control of fire. The jury is still out on whether Homo erectus was a direct ancestor of modern humans, or whether it was an evolutionary offshoot, but one way or another, we stop seeing Homo erectus in the fossil record sometime between 140,000 and 500,000 years ago.

Scientists are therefore left with a crucial conundrum: what happened to H. erectus? Maybe they simply evolved into another species of human that eventually evolved into us, or maybe they were a dead end that went extinct for other reasons.

A new theory that's making headlines, proposed by archaeologists out of the Australian National University (ANU), falls squarely in the latter camp, that Homo erectus was a dead-end species.

And the reason why they went extinct, according to this theory? H. erectus was lazy.

"They really don't seem to have been pushing themselves," said Dr. Ceri Shipton, lead researcher behind the new theory, in a press release. "I don't get the sense they were explorers looking over the horizon. They didn't have that same sense of wonder that we have."

Hints of a poor work ethic

Shipton and colleagues base this "sense" on data collected from a single known H. erectus archaeological site in central Saudi Arabia. According to their analysis, the ancient humans that used this site showed a poor work ethic in how they collected and manufactured their stone tools.

"To make their stone tools they would use whatever rocks they could find lying around their camp, which were mostly of comparatively low quality to what later stone tool makers used," Shipton explained. "At the site we looked at there was a big rocky outcrop of quality stone just a short distance away up a small hill. But rather than walk up the hill they would just use whatever bits had rolled down and were lying at the bottom."

He continued: "When we looked at the rocky outcrop there were no signs of any activity, no artifacts and no quarrying of the stone. They knew it was there, but because they had enough adequate resources they seem to have thought, 'why bother?'".

By utilizing these "least-effort strategies," Shipton surmised that Homo erectus wouldn't have been able to adapt to a rapidly changing environment, let alone compete with other emerging, more ambitious humans such as Neanderthals and Homo sapiens.

It's a bold claim about the demise of a species that was able to survive for well over 1 million years. (By comparison, Neanderthals lived for roughly 400,000 years; Homo sapiens, still going strong, have only been around for 200,000 years at most.)

Not so fast

Needless to say, it's also a conjecture that's sure to drum up its fair share of criticism. The theory, based on analysis from a single archaeological site, fails to take into consideration the overwhelming bulk of evidence that might just as easily speak to H. erectus' ambitious, curious streak. For instance, they were the first human species to rapidly spread across the Old World, control fire, and develop complex hunter-gatherer social structures.

The theory also fails to consider that a "least-effort strategy" might, in some contexts, be evidence of highly rational, adaptive behavior. Least-effort strategies conserve energy, which could be a life-saver in an environment where resources are limited or diminishing, such as what Shipton and colleagues claim were the conditions at this site.

And who knows, perhaps spending less time climbing hills to collect rocks freed these ancient humans up to introspect, to think; to master the use of fire, for instance.

Homo erectus was, by most measures, a highly successful species. If they were lazy, we might want to reconsider the adaptive advantages that laziness must have played in the story of human evolution.

More than likely, though, the forces that caused H. erectus to go extinct were far more complex than this theory can explain. Theorists will need to do more heavy lifting before this mystery will ever be conclusively put to rest.