Did huge fires several million years ago force a transition from forest to savanna in northeast Africa? It’s a tantalizing thought, as such fires have been seen as a possible factor in driving the emergence of bipedalism in our remote ancestors. Adrian Melott (University of Kansas), who looks at the question in a new paper in the Journal of Geology, notes that our precursors would have adapted to such massive changes to their habitat, evolving to support life amidst the abundant grasslands that had replaced their former tree-filled environments.
The conjecture about early hominins is receiving a lot of attention, but it plays only a small role in this paper, which focuses on the linkage between supernovae activity and the period in question. Just how do we make the call on a nearby supernova? Melott has been studying the question for some time, and refers back to 2016 studies of ancient seabed deposits of iron-60 isotopes that appeared in Nature. At that time, two supernovae events, one from 1.7 to 3.2 million years ago, the other from 6.5 to 8.7 million years ago, were identified via iron-60 deposits.
The new study points to the accumulating evidence for supernovae in the vicinity of the Earth, finding the strongest signal at 2.6 million years, and notes that this may represent a series of supernovae beginning as far back as 7 million years that Melott relates to the clearing out of the Local Bubble, which is a cavity in the interstellar medium in the Orion Arm in which our own Local Interstellar Cloud is located. Supernovae are thought to have emptied this cavity.
Image: Supernovae bombarded Earth with cosmic energy starting as many as 8 million years ago, with a peak some 2.6 million years ago, initiating an avalanche of electrons in the lower atmosphere and setting off a chain of events that conceivably ended with bipedal hominins such as homo habilis. Credit: NASA.
In the paper, Melott and colleague Brian Thomas (Washburn University) assume a supernova occurring 2.6 million years ago at a distance of 50 parsecs, or 163 light years, a time after the Local Bubble had already formed. The authors compute the extent of cosmic ray propagation and consider effects on terrestrial life, with particular regard to muons, the highly penetrating elementary particles that are the primary component of cosmic ray irradiation at the surface.
A key result is increased ionization of the atmosphere as a result of supernovae cosmic rays, with ‘electron avalanches’ set off that are the primary initiator of lightning (a still controversial idea, though one the authors cite recent evidence to support). From the paper:
A 50 times increase in atmospheric ionization in the troposphere would clearly make the breakdown and electron cascade much easier, and one could expect a great increase in lightning (e.g. Erlykin and Wolfendale, 2010). Furthermore, the originally isotropic distribution of cosmic rays would result in showers which are much more preferentially vertical, due to the variation in atmospheric column density with angle. So, not only would lightning be enhanced, but cloud-to-ground lightning should be preferentially enhanced. The theory of lightning initiation is not well-developed, and we cannot say that a 50-fold increase in ionization would lead to a 50-fold increase in the number of lightning events. However, the potential is there for a large increase.
As the paper notes, lightning is the main initiator of wildfires (excluding humans), and an increase in lightning strikes would be expected to induce large numbers of wildfires. The conversion of forest to savanna, a worldwide phenomenon beginning about 7 million years ago, may have been caused by the increase in wildfires associated with this period, as found in soot and other carbon-related sediments. Thus the notion, not emphasized in the paper, that supernovae could have played a role in human evolution. Here’s Melott on the matter:
“The observation is that there’s a lot more charcoal and soot in the world starting a few million years ago. It’s all over the place, and nobody has any explanation for why it would have happened all over the world in different climate zones. This could be an explanation. That increase in fires is thought to have stimulated the transition from woodland to savanna in a lot of places — where you had forests, now you had mostly open grassland with shrubby things here and there. That’s thought to be related to human evolution in northeast Africa. Specifically, in the Great Rift Valley where you get all these hominin fossils.”
Supernovae are not frozen in time, so the logical question is whether events like these could occur in our future. A possible future Type II supernova (resulting from the collapse of a massive star and subsequent explosion) can be found in Betelgeuse, which may light up the skies sometime in the next million years, according to current estimates. But at 200 parsecs, it is not likely to cause major consequences. Melott and Thomas see solar events as far more concerning as we consider solar flares in the context of our technological civilization.
The paper is Melott and Thomas, “From cosmic explosions to terrestrial fires?” published online by the Journal of Geology 28 May 2019. Abstract / Preprint.
Of course, a Type Ia Supernova may occur somewhat closer to the Solar System, say from a binary white dwarf.
Yes, and those SN Ia are the main producers of Fe. All it takes is a binary with one white dwarf accreting material from an aging companion swelling past its Roche lobe. These types of binary systems are not that uncommon.
First, I’ll say that I have not read the paper and do not plan to do so. Even so I have to point out a couple of items.
One is trees versus grassland. When you burn down a forest you do not get a field of grasses, you get another forest. This is determined by soil chemistry. I have personal experience with this since I grew up in grassland and have lived for some time in forest (though a lot of it has been cleared). Trying to grow a tree in grassland is like try to pound a round peg into a square hole. It can be very, very difficult getting the tree to survive to maturity. Where I now live it takes (now banned) massive chemical applications to keep the native vegetation from killing your nice grass lawn. Trees in particular tend to sprout everywhere, much to my dismay. How does upper atmosphere ionization change soil chemistry?
Second, cloud-to-ground lightning requires a potential difference between the cloud and ground (that ought to be obvious). Ionizing the atmosphere is unlikely to cause cloud-to-ground lightning since the potential to ground hasn’t changed. Lightning is preceded by massive charge transportation between ground and atmosphere, then something to initiate formation of a conductive path between them. With enough potential nothing can stop lightning from occurring, so the trigger is not too significant.
I think you are talking about a different timescale. Frequent forest fires might well change the local environment, making way for the decline of the forest in favor of other ecosystems. Your personal experience of [temperate] forest fires is not going to apply to all forest ecosystems, especially any that are already being stressed. We do know that paleo climate changes had impacts on ecosystems as pollen analysis of cores proves. We also know from charcoal deposits that human induced fires also changed ecosystems.
So the big leap is the hypothesized causal mechanism of supernova explosions causing global loss of forests and resulting in hominids with more erect postures to be selected for. Without reading the paper, and just going by the various articles I have read, I have to admit I am skeptical too. It smacks of the “one event” hypothesis that neatly explains subsequent events. I would want a lot more supporting evidence before accepting this as having any impact, let alone the proximate cause.
True. The planet had a lot of other remarkable things coincidentally going on between 3.6 and 2.6 MYA. Himalayan uplift “suddenly” diverted atmospheric moisture into Asian monsoons, resulting in drier and drier climate cycles in east Africa. The Rift Valley would have felt those dry spells in even more abrupt 20,000 year pulses — by evolutionary timescales — as the planet’s eccentricity, obliquity and precessional gears turned. Those changes alone would have exerted a great deal of stress on our ancestors. Our forest-friendly predecessors, Australopithecines, didn’t make it.
So if the world’s paleontological record was going to record a great era of lightning induced forest fires, that would have been an excellent time for it. And if this fascinating astronomical explanation does bear up, it will fit the complex context of the times.
Either way, it’s fair to say we’re out of the woods now.
Perhaps the global fires suppressed trees and most plants through prolonged period of reduced sunlight? Grasses then reclaimed the vast barren areas more rapidly than trees. Herds of wildlife could have later maintained the savannahs, or perhaps there a tilt, axis change increasing aridity in favor of grasses. Perhaps increased CO2, precipitation from global fires played a role, exceeding levels desired by larger fauna but giving the grasses an advantage.
Supernovae add yet another aspect of complexity to the formation of humans.
Anaerobic archaea ingested aerobic alphaproteobacteria, the latter becoming mitochondria and conferring aerobic status. The host DNA was segregated to the nucleus, forming eukaryotes.
Eukaryotes evolved cellular aggregates, which acquired new properties like bacterial biofilms, but beyond that showed specialization in both cellular physiology and morphology of multicellular organisms.
A circulatory system for body fluid, a compound (hemoglobin) to ferry oxygen, a packaging system (red blood cells) to allow more hemoglobin in the fluid with lesser effects on viscosity and a gas exchange mechanism (lungs) allowing greater efficiency than with an aqueous medium, all contributed to body size not constrained by diffusion of gas or liquid from the exterior.
A continuous canopy of trees fostered brachiation, locomotion by hanging from overhead branches, resulting in three axes of movement in the shoulder. It also demanded binocular vision to accurately judge the distance of the next branch.
This is where the supernovae could have initiated the conversion of forest to grassland, promoting bipedality. This freed the upper extremity to reach, grasp, hold, carry, throw, etc. The configuration of the head was altered with the face facing towards the new forward. Control of fire, with cooking of food, led to shrinkage of teeth, which together with the new configuration of the head on the neck resulted in alterations in the airway and vocal mechanism conducive to the modulation of sound into speech.
Manipulation and modification of “sticks and stones” may have contributed to an adequate length of the thumb for a strong pinch, and binocular vision would have helped such manipulation.
Speech contributed to formation of community and society; this and stereoscopic manipulation of objects could have contributed to brain development.
So here we are.
So where are the remanents of those supernovae?
Excellent question Antonio. A few million years is a short time ago, astronomically speaking.
Suppose that global changes recorded on the planet Earth are consequences of evolution our Sun and bounded to the Sun cosmic objects (planets, satellites, asteroid, comets etc.) The distant Super Novas leave only records with minimal impact on the Earth environment.
By the way, same thing related to modern politically charged stereotype – “Global Climate warming due to human industry”…
P.S. I suppose it is possible to test in physical lab the key element of “Super Nova hypothesis” – lighting rate increasing with rise of atmosphere ionization level, and even build some graph the shows this dependency, I will not be surprised , if this experiments has been done already long time ago…
Remember, only YOU can prevent supernova-induced wildfires.
I’ll get my rake…
“The theory of lightning initiation is not well-developed…”
You said it, buddy! Lightning, and static electricity in general, is a most mysterious subject. Why does rubbing two dissimilar materials together cause them to be opppositely charged, often very strongly? As Feynman commented, nobody really knows. Why do you sometimes get glowing spheres floating about during lightning storms (ball lightning)? Some of them last for tens of seconds. Nobody has the faintest idea of a physical mechanism.
The fires 7 million years ago are the result of higher Co2 levels in that period which average 500 parts per million. Today’s Co2 levels at 415 parts per million take us all the way back to the Pliocene. Hotter weather result more dry, plants and flammable conditions and the potential for more forest fires as already written in another post.
Do you mean higher oxygen (O2) levels back then?
I think you got your cause swapped with effect Geoffrey, but yes, a worldwide spike in wildfires would obviously drive CO2 levels higher.
The Fe-60 ocean sediment layers together with the local bubble are compelling evidence of somewhat nearby SNs, even if the remnants can’t be located at present. For everyone’s info here’s what wikipedia has on Fe-60:
“Iron-60 is an iron isotope with a half-life of 2.6 million years, but was thought until 2009 to have a half-life of 1.5 million years. It undergoes beta decay to cobalt-60, which then decays with a half-life of about 5 years to stable nickel-60. Traces of iron-60 have been found in lunar samples.
In phases of the meteorites Semarkona and Chervony Kut a correlation between the concentration of 60Ni, the granddaughter isotope of 60Fe, and the abundance of the stable iron isotopes could be found, which is evidence for the existence of 60Fe at the time of formation of the solar system. Possibly the energy released by the decay of 60Fe contributed, together with the energy released by decay of the radionuclide 26Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of 60Ni present in extraterrestrial material may also provide further insight into the origin of the solar system and its early history.
Iron-60 found in fossilised bacteria in sea floor sediments suggest there was a supernova in the vicinity of the solar system approximately 2 million years ago. Iron-60 is also found in sediments from 8 million years ago.”
I can believe that Iron 60 in fossilized bacteria might be from a supernova, but it couldn’t have been too close. It had to be thirty or more light years from us, otherwise life would have become extinct or there would be mass extinctions. I just don’t think that such a supernova thirty or more light years from us could start forest fires.
The Co2 level of 500 part per million of that Miocene, 7 million years ago was not caused by forest fires, but was the result of a higher volcanic activity in the past and higher Co2 levels in the past the further back one goes, the higher were the Co2 level and the hotter the average global temperature. There were no polar ice caps at that time in the Cretaceous, Jurassic, and Triassic. The CO2 was much higher in the Cretaceous than in than 7 million years ago. It took a long time for the carbon cycle, and trees etc to take the Co2 out of the atmosphere so it was cooler in the Miocene but still hotter than today.
And if higher CO2 and resulting higher temperatures was associated with both a seasonal drier climate and denser vegetation, just as we are seeing in the US SW region, there will be more fires.
The Miocene saw the appearance of grasslands as there was a drying out during this period. The African Savannah predominated by 2 mya and associated with the appearance of humans.
So perhaps a favorable climate with suitable ecosystems for fires appeared. Whether supernovae triggered more fires, I think we need more data. I would want to see biome changes that appeared after the expected supernovae events, indicating proximate cause.
Thank you for that explanation Geoffrey. I meant to make the same point about these SN not being close enough to have caused extinction events. It is indeed far from proved that such SN could have increased lightening on Earth near as much as this work suggests.