Was the rise of humankind written in the stars? A nearby star exploding 8 million years ago might have triggered more frequent lightning on Earth. Wildfires ignited by that lightning could help explain the rise of east African savannahs 鈥� which many researchers think provided a vital backdrop for the early evolution of hominins.
The rise of African savannahs, beginning about 8 million years ago, has long been a mystery to biologists.聽 They are dominated by plants called C4 grasses 鈥� but those grasses appeared 20 million years ago, long before they rose to dominance. Some botanists now wonder whether . Grasses bounce back quickly after a wildfire while trees are slower to recover, so frequent wildfires would have favoured the expansion of savannahs.
Now researchers led by at Washburn University in Kansas, at the Potsdam Institute for Climate Impact Research in Germany and at the University of Kansas have provided a possible explanation for the surge in wildfires 8 million years ago.
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Deep-sea sediments of that age contain a lot of iron-60, which is produced in massive stars. Its presence strongly suggests there was .
The team says a pulse of high-energy cosmic rays from the supernova must have swept across Earth. They calculated that some of the cosmic rays would have been energetic enough to reach the lower atmosphere and ionise particles in the air 鈥� a process that .
The researchers then used a global vegetation model to assess the effect of a short-lived increase in lightning 8 million years ago. They estimate it could have reduced tree cover in east Africa by about one-tenth, enough to give grasses a toehold and start forming savannahs.
鈥淚t鈥檚 one of these nice stories that all fits together, with the caveat that there鈥檚 some uncertainty in some of those linkages,鈥� says Thomas. He will in Seattle, Washington, later this month.
Does it add up?
The research has received a cautious welcome.
鈥淭he basic justification for this whole story is this radioactive iron-60,鈥� says astrophysicist at Louisiana State University. 鈥淎nd it really only could have been ejected by a supernova.鈥� The supernova would have been very bright, enough to 鈥渕ake a twilight sky out of the night sky鈥�.
The link between cosmic rays and lightning formation is appealing, says at the University of New Hampshire. 鈥淭he thinking is that perhaps cosmic-ray air showers can somehow carve a conductive path through the storms, thereby helping lightning get started.鈥�
鈥淭he聽cosmic ray model is plausible and reasonable,鈥� says Schaefer.
However, a study Dwyer published in August found limited evidence for the idea. He examined 74 days of cosmic ray records and found no link with lightning, although the sample size was too small to rule it out ().
Schaefer is not surprised. 鈥淚t鈥檚 hard to prove these things,鈥� he says.聽鈥淭here might be multiple things that can serve as inducements for lightning.鈥�
The fact is, we do not understand how lightning is triggered. 鈥淚 find it amazing that something so familiar can be so hard to understand,鈥� says Dwyer.
The plants they were a-changin鈥�
Botanists say the surge in wildfires 8 million years ago does need explaining. 鈥淭here is a puzzle for me as to where the additional fire came from,鈥� says at the Royal Botanic Garden Edinburgh, UK. 鈥淭he idea that increased lightning may have pushed things over the edge is intriguing.鈥�
The remaining question is the degree to which the formation of savannahs encouraged hominin evolution. The idea was first mooted in the 19th century, but in the 21st century some researchers have attacked it. There is evidence that early hominins, particularly the 4.4-million-year-old Ardipithecus ramidus, lived in wooded environments.
But other scientists say . Researchers like at Complutense University in Madrid, Spain, say . 鈥淎ll hominins found until now are associated to savannah biomes,鈥� he says.
