Scientists have sequenced the genome of a 4,500-year-old man in Africa for the first time, a difficult achievement since the hot climate has made DNA difficult to recover from old remains.

The breakthrough was made using the skull of a man buried face down in a cave in the southern Ethiopian highlands, according to the study in the journal Science.

The cave was cool and dry enough to preserve his DNA for thousands of years, said the study, which noted that previous ancient genome analysis has been limited to samples from northern and arctic regions.

While the man died before a wave of migration back into Africa from western Eurasia around 3,000 years ago, his genome showed that this migration "was up to twice as significant as previously thought, and affected the genetic make-up of populations across the entire African continent," said the research team in a statement.

This event, known as the "Eurasian backflow," occurred when people from regions of western Eurasia such as the Middle East and Anatolia suddenly flooded back into the Horn of Africa.

By comparing his ancient genome to DNA from modern Africans, researchers found that East African populations today have as much as 25% Eurasian ancestry from this event.

Beyond the region, African populations across the continent can trace at least 5% of their genome to the Eurasian migration.

Researchers believe their findings show that the massive wave of backflow migration was far bigger than previously thought and may have amounted to a quarter of the population of the Horn of Africa at the time.

"With an ancient genome, we have a direct window into the distant past. One genome from one individual can provide a picture of an entire population," said Andrea Manica, of the University of Cambridge's Department of Zoology, a senior author of the study.

"The question is: what got them moving all of a sudden?"

The cause of the mass movement remains a mystery.