Exploding stars didn’t cause human bipedalism

Earlier this week, a short piece by Ian Sample, Science Editor of The Guardian, caused my Twitter, Facebook, and email inbox to simultaneously blow up with concerned inquiries from friends and family about whether exploding stars really had caused human bipedalism. The story spread quickly. As of this writing, the title alone gets me 3180 hits on a google search, and an endless stream of tweets.

My personal favourite Lamarckian follow-up so far: “B/c when the ✨ exploded it was so mind-bogglingly insane and epic that everyone was super motivated to give it their all to be able to see that ish from a better angle, and then once that view was achieved, they never went back 👁‍🗨. The end”.

Amid a flood despondent posts and tweets from fellow anthropologists and archaeologists, and some entertaining ones from astronomers, I started asking myself how such a SciComm train wreck of global proportions could have happened. There are a number of important lessons to be drawn from this episode. Disasters of this magnitude are generally not the result of a single failure, but of a sequence of related ones.

What the paper actually says

The original paper on which the Guardian piece is based, by astronomers Adrian L. Melott and Brian C. Thomas, has two very distinct components. The first claims that a number of nearby supernovae started modifying global wildfire regimes on earth about 7 million years ago by affecting levels of atmospheric ionization, and thus frequency of lightning strikes. This could have led to the emergence of more savannah environments.

This is the bulk of the paper. I am not terribly well equipped to evaluate their contentions about the bunch of supernovae, but paleo-environment is closer to my bailiwick, and the idea that fire regimes were affected makes sense. Whether the change in fire-regime would have led to the emergence of Savannah is an interesting hypothesis, but definitely needs lots of testing. The long-term impact of changes in fire-regimes on local environments is not well understood, to say the least.

The second component  occupies fewer than five lines of the ten page paper. Without even explicitly noting that human ancestors most likely became bipedal around the time of the change in fire regime, the authors suggest that “The conversion from woodland to savannah has long been held to be a central factor in the evolution of hominins to bipedalism, although more recent thinking tends to view it as a contributing factor (Senut et al., 2018). Thus, it is possible that nearby supernovae played a role in the evolution of humans”.

That is the sum total of the paper’s discussion of the relationship between supernovae and bipedalism. There is no evidence presented, no attempt is made to evaluate the hypothesis, and the mechanism proposed is sketched out in its absolutely barest outlines. It would need a lot of working-out before anyone could even start to think about designing a test.

The last few lines of the paper raise the possibility that a similar event might happen in the future. Since there is a possibility that nearby supernovae might have affected human evolution in the past, isn’t it reasonable to ask ourselves how we would be affected in the present if this happened again? Yes it is.

How it went supernova

I read this as a ham-handed attempt to introduce some note of contemporary relevancy by scholars who are used to having their topic considered exotic and useless. I mean, how many people, without an explanation, would worry about how the study of ancient supernovae, near or far, affects their daily lives?

Unfortunately, the bipedalism claim, while completely extraneous to the main thrust of the paper, is included in the abstract. This is the first failure. For all you students and early career researchers out there, this is an important lesson. Make sure your abstract sticks to your main point.

For all you science journalists out there, when scanning abstracts, before highlighting them to your millions of readers, please make sure that they actually reflect what is in the paper they purport to summarize. This is the second failure.

In the Guardian, Sample picks up on the completely extraneous and speculative bipedalism claim, and puts it at the centre of his coverage, generating a sensational headline. This is the third failure.

Worse, when contacted by Sample, one of the authors (Melott), instead of sensibly contextualizing and explaining that the bipedalism claim is only an illustrative speculation about why we should be interested in supernovae, actually doubles down with a quasi-lamarckian caricature of the 19^th century savannah hypothesis for the origin of human bipedalism: “When the forests are replaced with grasslands, it then becomes an advantage to stand upright, so you can walk from tree to tree, and see over the tall grass for predators”. This is the fourth failure.

The fifth failure in this disaster chain is that Sample apparently didn’t talk to anyone who actually studies the evolution of bipedalism. I know some. They are generally personable and helpful. They would have been happy to comment on the supernova paper. Of course, that might have changed the outcome of the story.

Supernovae did not cause bipedalism

I can’t leave my readers today without reassuring them that supernovae did not, in fact, cause human bipedalism. Many different interacting random mutations caused bipedalism. As always with evolutionary problems, the why is both less important and less tractable than the how.

What caused bipedalism to become the normal human pattern (the how) is an entirely different question than the why. It’s possible that nothing at all caused most humans to become bipedal. It could have been a neutral trait that drifted to fixation in the population. In other words, it could be that by mere chance, the trait spread until most humans exhibited it. It could have been as simple as sexual selection.

It is quite likely, however, that selective pressure was involved. Those of our ancestors who initially became bipedal had some kind of advantage over the others and the trait spread that way. Since bipedalism evolved over a long period of time and in diverse environments, it is likely that a complex series of selective advantages was at work, rather than just one massive advantage.

It is possible that the emergence of savannah environments in eastern and southern Africa played a role. It is possible that an increase in the frequency of wildfires in Africa played a role in the emergence of savannahs. It is possible that an increase in the frequency of lightning strikes played a role. It is possible that the increase in frequency of lightning strikes was caused by an increase in atmospheric ionization. It is even possible, that ancient nearby supernovae played a role in increasing atmospheric ionization on earth between 7 and 3 million years ago.

Those are a lot of possibles between supernovae and bipedalism. There are many interacting mechanisms, genetic, environmental, behavioural, and demographic, over millions of years, and over an entire continent.

I like a good just-so story as much as the next evolutionary anthropologist, and this is certainly a fun one to read and think about. But at this point, it is an off-hand, highly speculative hypothesis. If anyone wants to properly evaluate it, let me know. I support you all the way.