Are humans top predators? We were before our brains shrunk

One of most exciting studies I’ve read this year just popped up in the journal Quaternary and was summarised on Phys.org [1]. Researchers suggest that human brain size has changed in correlation with our hunting ecology. While it’s considered a “novel theory”, it makes a lot of sense. Are humans top predators? We used to be.

Regular readers of this blog will know that I constantly refer to the thermodynamicThermodynamics are at the heart of our understanding of ecosystems and not an altogether difficult concept to grasp but one that isn't widely taught to ecologists. Basically, all life on Earth, is derived from the Sun's heat. This renewable energy source constantly bombards ecosystems with energy but they would overheat, if it wasn't for the absorptive capacity of food webs. More underpinning of Earth’s ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More and how this is all driven by animals–humans being one of the latest in a long line. Although Einstein published his views of the famous second law in 1904, for ecologists, this represents a new way to see the world functioning and for me, this new study of human-animal interactions may fill a gap in the evolutionary narrative that I have been struggling with for some time.

“It is the only physical theory of universal content, which I am convinced, that within the framework of applicability of its basic concepts will never be overthrown.”

Albert Einstein, 1967

Before the first humans (genus = Homo) evolved about 2 million years ago, our planet had been going through millions of years of evolution that resulted in animal-driven ecosystems only relatively recently. What I’ve struggled to comprehend is how Homo sapien managed to become the next most likely animal to survive on Earth, when the ecosystems would have already been stacked full of creatures about 10 million years ago, so almost every piece of free surplus energyThe energy of a system that is emitted as waste and is not part of ecosystem processes. There is always some free surplus energy as this creates the basis for evolution where new species exploit gaps in the ecosystem where free energy becomes available. Surplus energy can occur as a result of disruption or disturbance. When free surplus energy reaches More was used up.

Modern humans are omnivores: we eat a mixture of plants and meat. The diversity of our diet means that we exist at different parts of the food chainA single thread in a food web illustrating the chain of animals that eat each other. At the base of the food chain are small high-energy (fast metabolism) animals and at the other end large low metabolism animals. An example would be whales eating krill that eat plankton that eat algae. Or lions that eat gazelles that eat grass. More. On the trophic pyramidThe gradual reduction in energy content, increase in body size and reduction in number of animals, that occurs the higher you go up the food chain. At the base of the pyramid are a vast number of high-metabolism, tiny creatures and at the summit, are the top predators. To be stable, the pyramid has to have creatures at all levels. More (which measures where animals sit in energy terms), we are at about level 2.5 … about half way up. That’s an average, given we eat plants (level 1) and mostly meat at level 3-5. It doesn’t make sense for an animal of such stature and sophistication, to evolve in Earths late stage, if it was to be a generalist from the start.

You see, without energy to fuel a large brain and body, you can’t exist. Ecosystems are the mechanism (created by animals) that stabilise our planet’s thermodynamic forces and minimise what’s known as entropyThe degree of disorder or chaos in a system, most often used to describe thermodynamic energy but also used the behaviour of information. All else being equal, physics determines that all matter and energy moves towards chaos, therefore biological systems are in a continual state of battling against entropic forces in order to remain stable. The most stable ecosystem is More, the force that eventually makes systems fall apart but the same force also enables us to extract energy from the food we eat. This poses a conundrum. We need ecosystems to exist but the very same processes that create them, reduce our ability to extract energy.

This is the reason why the most diverse and stable ecosystems are extremely nutrient-poor.

It’s also why, survival isn’t of the “fittest”, it’s of the likeliest.

It takes about 25 million years for ecosystems to become fully saturated with animals at which point there is very little entropy potential, so there is limited scope for a new large mammal like us to evolve … unless of course, the system becomes unbalanced.

Ecosystems are never stable though, they are only ever in a steady stable-state(of an ecosystem) where free surplus energy is minimised, where there is maximum entropy production and minimum waste. In such a system, there is expected to be relatively small fluctuations in atmospheric and other chemistry and where disruption or disturbance occurs, the resulting changes can be absorbed quickly by a succession of new plants and animals that enter to fill More. There is always free energy on the outside, like the wind that gently rocks a tree. What if the very animals our neolithic ancestors hunted to extinction were the driver for that imbalance?

Large animals literally have a very big footprint on the Earth so the average lifespan of higher vertebrates is only between 200,000 and two million years[2]. Trampling the ground, ripping up vegetation and knocking over trees, changes things. The very basis of your existence and how all animals behave, is to alter one’s living environment and behaviour, to bring the two into some kind of alignment. Animal brains are the computational and storage device that lets us do this … which explains why the researchers found brain size changed. When we’re spearing large, slow-moving animals that are easy to find, we only need a relatively small storage disk for the information. As we became dependent on chasing smaller animals that were harder to find, our brains needed more capacity and since we have had agriculture they’ve shrunk back–and most likely will get smaller still, with the advent of supermarkets and fast food restaurants!

An animals’ survival has always been a fine balance between disrupting its environment increasing entropy (releasing more energy with which to breed and multiply) and maintaining stability, in order to survive longer as a species. Ironically, the successful spread of large megafauna across the Earth might have compromised their own survival but we may have them to thank, as they became the trigger that enabled humans to evolve in the first place.

While we tend to look for physical explanations for evolution and ecology, such as drying of the African continent, ice-ages etc., this overlooks the fact we exist among animals. The biodiversityWhat is the definition of biodiversity? When we ask, what is the definition of biodiversity? It depends on what we want to do with it. The term is widely and commonly misused, leading to significant misinterpretation of the importance of how animals function on Earth and why they matter a great deal, to human survival. Here I will try to More processes we depend on for a habitable Earth, are created by the interaction of all animals and we are part of the same. We are an animal.

It makes sense that human beings evolved because we were the most likely to bring the destabilising effects of megafauna back under control. For hundreds of millions of years, entropy forces have conspired time and again, to create new wildlife regimes to slowly bring things back to a steady state.

In high-functioning pristine ecosystems, the like of which would have existed two million years ago when our first ancestors evolved from the great apes, adaptation pressures were much greater because the energy was precisely distributed and advantaged only the most specialised. For humans to have even a slight chance of existing, would have required other very large animals, to transfer(of nutrients) the thing that sets animals apart from plants, is that they can move. Some of the biggest migrations on Earth every day, are the movement of insects like caterpillars, from the stem of a plant to a leaf and back, before turning into butterflies and transferring the energy elsewhere. Large-scale migration of grazing animals and migratory songbirds moves More, amplifyAmplification (of nutrients and energy). Animals consume plants and other animals and in doing so, reintroduce important energy-containing nutrients back into the environment, at even higher concentrations and in patches. Amplification of energy is driven by migration and happens at every scale, from insects moving daily in and out of your vegetable patch, to African wildebeest herds and the seasonal More and concentrate nutrientsEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More to an extreme level. How else would we have found sufficient surplus energy to evolve, than by hunting? In this case, the megafauna themselves became the concentrated source of nutrients (food) that bore an entirely new genus of primate.

So humans didn’t begin as generalists, we became generalists after our generic ancestors wiped out megafauna and altered the landscape. We have a tendency to think of “humans” appearing 300,000 years ago but the concept of a species is entirely subjective. Scientists have made that decision based on certain genetic thresholds, for example. What if we are simply the latest incarnation of a primate that had its stronghold a million years or more ago? What if the adaptation to an omnivorous lifestyle is the final stages in a species’ decline from existence? Is it coincidence that the genus Homo appeared two million years ago, which is about exactly the maximum lifespan of an average higher vertebrate on Earth?

After all, we continue to alter the environment ourselves, to suit our new and ever-changing lifestyle. In a way, we continue to adapt to conditions of our own making but this isn’t necessarily good news. Like the megafauna, we may become victims of our own success, if we force ecosystems into chaos(Of energy and ecosystems). Ecosystems are thermodynamically driven. Disorder occurs when energy dissipates and becomes more chaotic. For example, the release of hot air into the atmosphere results in that energy is freer to disperse (maximum entropy). The opposite is true when energy is locked into biological processes, when it is stored inside molecules (minimum entropy). Stability in ecosystems occurs More.

The decline of megafauna and the short lifespan of species with a large footprint on the Earth should send the strongest message to humanity–disrupt things too much and Earth will find a way to bring your species to its natural conclusion. It’s another example of how co-dependent we are on other animals, to create a habitable Earth and why the conservationWhy is animal conservation important? Animal conservation is important, because animals are the only mechanism to create biodiversity, which is the mechanism that creates a habitable planet for humans. Without animals, the energy from today’s plants (algae, trees, flowers etc) will eventually reach the atmosphere and ocean, much of it as carbon. The quantity of this plant-based waste is so More of endangered species, protection of pristine habitat and rewilding of broken landscapes is essential for all our futures.

1. Ben-Dor, Miki; Barkai, Ran. 2021. “Prey Size Decline as a Unifying Ecological Selecting Agent in Pleistocene Human Evolution” Quaternary 4, no. 1: 7. https://doi.org/10.3390/quat4010007
2. Ehrlich, P.R., A.H. Ehrlich, and J.P. Holdren, Ecoscience: Population, Resources, Environment. 1977: W. H. Freeman.