We may have misunderstood the mother of all extinctions. The gargantuan Permian extinction has been blamed on massive volcanic eruptions that killed swathes of organisms, but the eruptions may instead have had an insidious effect: sterilisation. Organisms may not have been killed outright, but if they could not reproduce their species were still doomed.
Almost all complex life died 252 million years ago at the end of the Permian period. The causes have long been debated.
About a decade ago geologists began noticing something odd about fossil pollen from the time. An unusually high number of the pollen grains were .
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That might be because the volcanic activity at the time released ozone-destroying chemicals into the atmosphere. As a result, more of the sun鈥檚 harmful ultraviolet-B (UV-B) radiation reached Earth鈥檚 surface. The UV-B would have stressed plants, particularly the abundant conifers and seed ferns, which .
Radiation danger
To mimic this environmental upheaval, , Ivo Duijnstee and at the University of California, Berkeley exposed 18 dwarf conifers to elevated UV-B levels for 56 days. The tiny trees produced elevated levels of malformed pollen, as predicted.
But something unexpected happened. Although the trees survived UV-B exposure, they were all rendered infertile throughout. The pines made seed cones, but these died before they grew large enough to be fertilised. 鈥淭he shrivelled-up seed cones were a big surprise,鈥� says Looy.
This was true even of cones that were shaded from direct exposure. 鈥淎s long as just part of the tree is exposed to the radiation, it鈥檚 not going to be happy enough to reproduce,鈥� says Benca.
This suggests a new scenario. Perhaps there wasn鈥檛 a sudden mass die-off triggered by volcanic activity. Instead, the resulting UV-B may have sterilised most land plants. There seem to have been pulses of volcanic activity over thousands of years, so this may have happened over and over. Generations of plants died of old age, without leaving enough offspring to replace them. Eventually, forests and the animals living in them declined.
No more children
鈥淚t鈥檚 not necessarily about the catastrophic killing of individuals,鈥� says Duijnstee. 鈥淭here鈥檚 maybe something else going on, such as a drop in fecundity so species cannot cope.鈥�
鈥淚 think their conclusions are really robust,鈥� says at Trinity College Dublin in Ireland.
The study is the first to examine whether the environmental upheaval could have affected the female side of production 鈥� seed cones 鈥� and not just the male side represented by pollen, says McElwain. 鈥淚t鈥檚 amazing no one has thought to look at that.鈥�
However, at the Natural History Museum in London isn鈥檛 convinced, pointing out that the experiments only used one species of conifer. 鈥淎nd even if it happened, it鈥檚 irrelevant for marine extinctions,鈥� because marine species were hit hard but UV-B cannot be responsible.
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