DNA study suggests ‘climate event’ contributed to collapse of empires
Examination of ancient bones by TCD scientists identifies genetic change in cattle herds 4,000 years ago
A modern day zebu, an arid-adapted breed which emerged in greater numbers 4,000 years ago as a suspected result of a dramatic drought, or ‘4.2 kiloyear climate event’. Photograph: Denis Doukhan/Pixabay
A study of ancient DNA from now extinct cattle breeds has revealed new evidence explaining the demise of the world’s first empires, and the part that may have been played by a drought that lasted for centuries.
Livestock herding began in the ancient Near East – covering much of what is now the Middle East – where civilisation emerged 7,000 years ago.
It underpinned the emergence of complex economies and then cities. Subsequently, it was in this region that the great empires of Mesopotamia and Egypt emerged.
The DNA from ancient bones extracted by an international team headed by geneticists in Trinity College Dublin has revealed how the prehistory of the region’s largest domestic animal, the cow, connects with these events.
The archaeozoologists deciphered the history of early bovines by sequencing 67 ancient genomes (the complete genetic make-up of species) from both wild and domestic cattle sampled from across eight millennia.
“This allowed us to look directly into the past and observe genomic changes occurring in time and space, without having to rely on modern cattle genetic variation to infer past population events,” explained Dr Marta Verdugo, who is based in TCD and lead author of their study published in the journal Science on Thursday.
Sequencing near eastern wild cattle known as aurochs – large wild cattle that inhabited Europe, Asia, and North Africa and are an ancestor of modern cattle – helped the team “unravel the domestication process of this most formidable of beasts”, she said.
Whereas their genetic similarity to early domestic cattle of Anatolia provides evidence of their origin in that region, it is clear different local wild populations also made significant additional genetic contributions to herds in southeast Europe and in the southern Levant, and to the distinctive genetic mix of European and African cattle populations today.
These earliest domestic cattle were Bos taurus, unrelated to zebu cattle which, with their distinct humped back, were part of herds that originated separately further to the east in the Indus Valley.
“However, a dramatic change occurred around 4,000 years ago when we detect a widespread, wholesale influx of zebu genetics from the east,” Dr Verdugo said.
The rapid incursion occurring at this point may be linked to a dramatic drought that was experienced across the greater Near East over many centuries, known as “the 4.2 kiloyear climate event”.
At this time, the world’s first empires in Egypt and Mesopotamia – which means “land between two rivers”, a vast region between the Tigris and Euphrates river systems – collapsed and breeding with arid-adapted zebu bulls may have been a response to changing climate by ancient herders.
“This was the beginning of the great zebu diaspora that continues to the present day – descendants of ancient Indus Valley cattle are herded in each continental tropics today,” said TCD professor of population genetics Dan Bradley.
“There is a great power in ancient genomics to uncover new, unforeseen tales from our ancient history,” he added.