Putting the life back in science fiction

Hobbits of the ATM?

No, I haven’t seen the latest offering Peter Jackson yet, but I will soon. Still, in honor of the latest, erm, extension of The Hobbit onto the big screen, I thought I’d pitch out an interesting possibility for the future of at least some of our descendents.

First, a definition: ATM isn’t the money machine. Rather, it’s an acronym for Anthropocene Thermal Maximum, which we’ll hit sometime after we’ve exhausted all the fossil fuels we’re willing to dig up into the atmosphere. If we blow off over something like 2500 gigatonnes of carbon, we’re going to be in the range of the PETM, the Paleocene-Eocene Thermal Maximum (Wikipedia link) about 55.8 million years ago, when global temperatures got as hot as they have been in the last 60 million years. Our descendents’ future will be similar, if we can’t get that whole carbon-neutral energy economy working.

One of the interesting recent findings is that mammals shrank up to 30 percent during the PETM (link to press release). The reason given by the researchers is that increased CO2 causes plants to grow more foliage and fewer fruits (in the botanical sense, so we’re talking fruits, nuts, grains, and all the other things we like to eat). This poorer nutrition led to smaller animals. I think there’s another possible explanation for the decrease in animal size.

My thought was that, if civilization crashes due to radical climate change into a PETM-type world, humans will be at the mercy of the elements, so it’s quite likely that future people will be smaller in size. Perhaps 30 percent smaller? Sitting down with the BMI graph and making a few assumptions, I found that the 30% smaller equivalent of a 71 inch tall male weighing 160 lbs is approximately 60 inches tall. Now, this is an interesting height, because it is the upper limit of pigmy heights in an interesting 2007 study by Migliano et al. in PNAS (link to article). Their hypothesis was that the evolution of pigmies around the world is best explained by significant adult mortality, which they adapted to by shifting from growth to reproduction earlier in their lives. The researchers found that the average age at mortality in pigmies is 16-24, and few live into their 40s. The major cause of death is disease, rather than starvation or accidents.

While I don’t know of any evidence of increased animal disease during the PETM, there is good evidence for increased plant disease and predation by insects (link), so it’s not much of a stretch to hypothesize that the animal dwarfing could have been caused by increased disease, decreased lifespans, and a resulting shift towards smaller body size and early reproduction.

So, here’s the idea: if we blow too much carbon into the air, and our ATM rivals or exceeds the PETM, at least some of our descendents will be the size of pigmies, due to the harsher environment (more disease, less medical care) favoring people who mature earlier and have kids as teenagers. They probably won’t be hobbits unless a hairy-footed morph takes off somewhere (perhaps in the jungles of Northern California?), but they will be technically pigmies.

It’s not the most pleasant thought, but if short lives and statures is troubling, the good news is that post-PETM fossils show that animal species regained their former size once the carbon was out of the air. And, according to Colin Turnbull’s The Forest People, life as a pigmy isn’t necessarily nasty or brutish, even if it’s short.


3 Comments so far
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Modern humans with technology will not be subjected to the same evolutionary drivers as wild animals. That some species became smaller may be due to any number of factors – food supply, predators, the physiology of heat regulation, even chance. Human evolution is, I believe, strongly sex-selection driven. As we know, the selected features are somewhat arbitrary. Maybe selection for small size would happen, but I doubt it, unless it is accompanied by relative enhancement of cognitive skills.

While you use pygmies as an example of a humans living in a hot climate, the majority of humans living in such conditions are not very small. At best, humans living near the equator may follow this model of size decline, but most will migrate to cooler climes (even Antarctica, where larger body size might still be advantageous.

Comment by alexandertolley

Thanks Alex,

I think we’ll agree to disagree. One thing is that current human evolution seems to be driven largely by things like tolerance to alcohol, grain based diets, lactose tolerance, and similar. It’s not clear if sex matters in evolutionary terms, first because sex standards vary quite a bit, but more because there’s no evidence that the sexiest people have more children.

As for the pigmies, it’s worth reading the Migliano paper, simply because they look at the evidence for all the hypotheses for why pigmies are shorter. Obviously, not every person in a hot climate is a pigmy. For example, the Turkana, from an even hotter desert, are among the tallest people in the world.

One issue with the PETM is that the temperature gradient between the equator and the poles radically flattened out. There are alligator fossils in Greenland and Alaska from that time, but there are also tropical rain forest fossils from within 10 degrees of the equator. The PETM leaf fossils in the study quoted above came from Wyoming. While I agree that people will migrate to the poles if we see major global warming, that doesn’t mean that the equator will become totally unlivable. Miserable yes, but not necessarily unlivable.

As for technology, that’s the critical question, isn’t it? At this point, our global technological civilization certainly looks a lot more fragile than our species is. Keeping it that way will be a challenge.

Comment by Heteromeles

I believe that there are groups classified as “pygmies” who are not tropical – can’t name any off the top of my head, but these would be Eurasian. The Migliano hypothesis makes the most sense, at least given our present state of knowledge. When survivorship decreases, best to mobilize lifetime investment from growth to reproduction as early as possible.
Haven’t read the Gingerich paper yet, but I wonder if the dietary explanation is the best. An increase in the availability of coarse fodder would select for larger, not smaller, animals.

Comment by Lars

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