Studded with five intact teeth, the jawbone reveals that our genus, Homo, appeared almost 500,000 years earlier than previously believed, after branching off from the more apelike Australopithecus genus that included the likes of “Lucy.”

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Has climate change made us who we are today?

A broken and fossilized jawbone found poking from the sediment of an East African hill is rewriting a significant chapter of human evolution — and adding weight to the argument that a hot and parched climate guided the development of our ancestors.

In two papers published Wednesday in the journal Science, researchers described the discovery of a 2.8 million-year-old jawbone in Ethiopia’s Afar regional state. Studded with five intact teeth, the mandible reveals that our genus, Homo, appeared almost 500,000 years earlier than believed, after branching off from the more apelike Australopithecus genus that included the likes of “Lucy,” perhaps the most famous set of skeletal remains.

The significance of this discovery, according to some researchers, is that it firmly fixes the origins of Homo in east Africa and fits the hypothesis that climate change drove key developments in a variety of mammals, including our early ancestors.

When Lucy roamed Ethiopia about 3.2 million years ago, the region enjoyed long rainy seasons that supported the growth of many trees and a wide variety of vegetation, according to researchers.

By the time of Homo’s first established appearance in the Horn of Africa, however, things had become much drier and the landscape had transformed into a vast, treeless expanse of grasslands with a few rivers and lakes, a scene similar to today’s Serengeti plains or the Kalahari.

It was an unforgiving climate when it came to survival.

But the hallmark of the genus that includes Homo sapiens is resourcefulness. Larger brains, the ability to fashion stone tools and teeth suited to chewing a variety of foods would have given our early ancestors the flexibility to live in an inflexible environment, researchers say.

“This early Homo could live in this fairly extreme habitat, and apparently Lucy’s species could not,” said Kaye Reed, an Arizona State University paleontologist who worked on both studies.

Researchers were unable to determine the fossil’s age directly. Instead, they used radiation dating to estimate the vintage of a layer of volcanic ash and crystals roughly 30 feet below the fossil, which is known as LD 350-1.

“The time period between 2 (million) and 3 million years ago is one of the least-well-understood in human-origin studies,” said co-author William Kimbel, director of Arizona State’s Institute of Human Origins.

Oddly, some of the natural forces that make it so difficult to find archaeological specimens of a certain age make it relatively easy to determine changes in Earth’s ancient climate.

Soil that is flushed out to sea by erosion or blown off the land by monsoon winds, will drift to the ocean floor and form vast layers of undisturbed sediment. These layers provide chemical clues to periods of dryness or abundant vegetation, researchers say.

Scientists have used these layers to assemble a “robust” record of ancient climate and hypothesized that it probably fueled human evolution.

The newly discovered jaw dates to a period of tumultuous climate change that included the arrival of the first ice age in the Northern Hemisphere, said Peter deMenocal, a paleoclimatologist at Columbia University who was not involved in either study.

The jaw’s discovery and an analysis of a mandible from Tanzania that was described Wednesday in the journal Nature have scientists reordering the branches of the human ancestral tree.

It is now clear that three separate species of Homo existed between 2.1 and 1.6 million years ago, although not all simultaneously: H. habilis, H. rudolfensis and H. erectus.

Scientists already know enough to say with certainty that East Africa is the birthplace of Homo, according to paleoanthropologist Donald Johanson, one of Lucy’s discoverers.