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The measles virus crossed over to people from cattle around 500 BC, supporting the idea that it could only get established as a human disease once large enough cities had developed. “It’s not proof, but it’s compatible with the notion that large cities might have provided the opportunity for it to emerge,” says Sébastien Calvignac-Spencer at the Robert Koch Institute in Berlin, Germany.
The measles virus evolved from the virus that causes rinderpest, a disease that used to be common in cattle and led to famines in Africa in the 20th century, until vaccination eradicated it by 2011.
Measles used to infect nearly all children until vaccination began. It is still a major public health problem in developing countries – and as a result of vaccine scepticism, there have been outbreaks in some Western countries too.
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It was already known that the measles virus evolved from the rinderpest one because they are so genetically similar, but it was unclear when it made the jump. Previous estimates were that it happened around AD 900. But that conclusion was based on analysis of fairly recent measles viruses.
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Calvignac-Spencer’s team found a preserved lung specimen in a Berlin museum from someone who died from measles in 1912. The researchers drew up a viral family tree by comparing this with 50 other virus genomes, either from recent measles cases, rinderpest or a related virus that infects sheep and goats. This dated the jump to 500 BC.
The analysis cannot tell us where in the world the crossover happened. But this earlier date roughly coincides with the emergence of cities of several hundred thousand people in China, India, North Africa and Europe.
From looking at the circulation of measles within island communities, we know it can’t survive for long in places with fewer than about half a million people. This is because it causes lifelong immunity, so once everyone has had it, there are no more hosts to keep it going. Only in larger communities would there be enough new, and therefore susceptible, babies being born for the virus to survive.
Microbes jumping from one species to another are a common cause of pandemics – as happened with coronavirus.
Journal reference: Science, DOI: 10.1126/science.aba9411
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