Who Killed the Megafauna?

It was us.

The Goddess Gaia is very fond of her elephants because they are one of the few large mammal species (“megafauna”) surviving on Earth after human beings exterminated most of them. She also knows whether humans will survive the perturbation they are creating; after all, they are part of the megafauna. But she won’t tell us.

For years, the debate has been going on: who (or what) killed most of Earth’s large mammals, (the “megafauna”) over the past 50,000 years or so? A fraction of the scientific community clings to the idea that climate change was the culprit. It is understandable, because most of us don’t like the idea that our ancestors behaved so ruthlessly. Instead, if it was the climate, it was an act of God (more exactly, of the Goddess), and hence it is not our fault.

The issue has political overtones, too. If our ancestors could do so much damage even with their primitive hunting technologies and small populations, imagine what we can do today to destroy the ecosystem! And, unfortunately, we are doing exactly that: destroying everything that can be destroyed. A recent paper by Svenning et al. weighs in, showing that our ancestors are the most likely culprit for the extinction of the megafauna.

It is a highly suggested paper. It is especially interesting in its discussion of how the removal of the megafauna was not without consequences on the whole ecosystem, and it was recent enough that these consequences are still reverberating in our times. A typical illustration of the fact that you cannot do one thing in biology.

The study has also several “deep time” implications. Look at this figure from the paper:

As often happens in these publications, the authors want to put too much data in a single figure, and the result is confusing. But just look at the continuous black curve at the top. It is the average maximum body mass of mammalians. Note that the scale is logarithmic: the last batch of mega-mammals included creatures weighing tens of tons, whereas, at the beginning of the Cenozoic, there was nothing walking around weighing more than 100 kg. These late mammals were comparable in mass to the spectacular Cretaceous Sauropods, although not as big as the largest ones. In this image from PaleontologyWorld you see the Pleistocene mammals compared to the Patagotitan mayorum, possibly the largest sauropod that ever existed, maybe weighing more than 50 tons.

It took some 50 million years to go from mammals weighing a few tens of kgs to several tons. Biological evolution, alone, can be much faster than that. What we are seeing here is the gradual adaptation of biological creatures to an environment that they were shaping and which favored huge beasts. Here is an interpretation of mine of what happened; it is based on what’s known of Earth’s ecosystem, but I understand that there may be different interpretations. Anyway, here is my take.

A concise history of the Cenozoic and its megabeasts

The Cenozoic Era started some 65 million years ago. It was a relatively quiet time in geological terms and, as a consequence, the geological emissions of CO2 were probably not sufficient to compensate for the biological and mineral sinks. The few LIPs (large igneous provinces) of the Cenozoic were small and caused, at most, an upward blip in the temperature/CO2 concentration. The result was a gradual decline in the atmospheric CO2 concentration and, consequently, a gradual cooling. (source of the image)

Once temperatures were sufficiently low, at the beginning of the Miocene (ca 22 My ago), an ice age started. As ice sheets expanded, they caused an increase in the planetary albedo, pushing down temperatures to even lower levels. At the same time, low CO2 concentrations favored those plants that could use the “C4” photosynthesis mechanism, which allows them to concentrate CO2 in their tissues. Trees cannot do that because it is incompatible with the mechanism of transportation of water from the roots to the leaves. So, forests died out, replaced by grassland. That also increased the planetary albedo, especially when covered with snow. More cooling.

Eventually, Cenozoic temperatures dropped as much as 15 °C, leading the ecosystem to the freezing times of the Pleistocene. The system's oscillations between ice ages and interglacials became a feature of the ecosystem. Maybe we could have seen a new episode of Snowball Earth, but that didn’t happen. (below, another reconstruction of the characteristics of the Cenozoic Era from Honisch et al, 2023)

Grazing herbivores co-evolved with grass, creating a wondrous grazing machinery that favored both, and created the condition for the formation of stable “mollisoil” that sequestered large amounts of carbon. The large size of the mega-herbivores was favored by the cold. In a cold climate, a low surface-to-volume ratio allows a creature to maintain a high internal heat without a large metabolic effort. At the same time, megaherbivores could crash their way through forests, leaving space for more grass to grow. Bigger beasts, more cooling. More cooling, even bigger beasts. The result was the huge biome that today we call the “Mammoth Steppe”

During the Pleistocene, it was not just herbivores that grew in size and in sophistication. Carnivores also became larger and both herbivores and carnivores developed sophisticated group behaviors for defense and attack. A species of primates with a high neuronal density in their cortex, the homo sapiens, won the arms race. They did that with a combination of stone tools, advanced social behaviors, and the possibility of adjusting their metabolism to group hunting by developing a turbo-charged body cooling system based on sweating. Almost nothing on Earth can beat humans in terms of endurance. So, they became the extremely effective hunters that they remain today. The result was one of the most spectacular examples of a “Seneca Collapse” I ever found. (figure below from Svenning et al.)

Practically all the largest mammals disappeared, except for a few that survived in Africa. Elephants, for instance, developed their huge ears as “radiators,” a cooling system that made them able to match human endurance. This point is well explained by Mera Te ‘Ai Enge’ite (2024). However, elephants may soon disappear, too, as the human perturbation of the ecosystem becomes stronger. Will any large mammals survive the human onslaught? We cannot say, and the Seneca decline may well go to zero, a value that may include human extinction.

The future of the megafauna.

Extinction may not be forever, at least in the sense that parallel evolution may lead to new species occupying the ecological niches left free by an extinct species while developing similar features. With the extinction of the dinosaurs 65 million years ago, an entire ecosystem of megabeasts was wiped out, but in some 50 million years, beasts of comparable size appeared again on Earth. Parallel evolution created them.

Now, even this new batch of huge beasts may disappear under a human-made catastrophe comparable to the burst of the Deccan traps that killed the dinosaurs at the end of the Mesozoic. But the results may not be so drastic. If humans go extinct soon, as it is likely to happen, the emission of greenhouse gases from fossil fuels will stop soon, and the biosphere may return to Pleistocene-like conditions in a relatively short time; maybe less than one million years. Some megabeasts could survive the transition.

Whatever the case, if Earth maintains its relatively low geological rate of outgassing, the land ecosystem will eventually return to the conditions of the late Miocene, when the largest megabeasts evolved. Even in the worst-case hypothesis, we will see again grasslands dominating the planet and huge herbivores roaming the plains. We might even see new hominins better integrated into the ecosystem, with the megaherbivores having learned how to avoid being exterminated, just like elephants did. Maybe elephants and humans can become friends. Only Gaia knows what kind of fauna will develop in these conditions. And she always knows best.

This post is strongly inspired by a paper by Gregory Retallack (2012)