Environment

The evolutionary history that made humans the ultimate invasive species

We’ve spread far beyond any other plant or animal, but what makes us unique?

September 9, 2020
illustrating human migration
Surviving the chaotic moments in Earth’s history may have given us the ability to take over nearly any habitat and out-compete native species everywhere we’ve gone. [Credit: CKSegarra | CC-BY]

Life has been quite favorable to us humans. A small group of hominids from Africa ventured into a big world filled with endless challenges. Since then, we’ve managed to spread far beyond what they knew possible. After our emergence as a unique species several hundred thousand years ago, humans have spread to every continent on Earth — a few of us have even lived in the great beyond of space. Although other species have spread widely, none come close in neither geographic breadth nor ecological impact, which begs the question: Why us? Was there a crucial event early in our evolutionary history that gave us the unique ability to dominate a planet teeming with countless other species?

In his book, “On Natural Selection,” Charles Darwin wrote of a “struggle for existence.” His observations on adaptations led him to speculate that even small advantages make a big difference: “Can we doubt … that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind?” 

Our early ancestors fit into that struggle. They had advantageous traits: walking upright, stone tool use, and large brain size — which enables communication and culture. But why did we get these traits, and are they the reason we’ve spread across the globe? 

In their book, “The Origin and Evolution of Cultures,” anthropologist Robert Boyd and cultural biologist Peter J. Richerson argue that “culture has made humans extraordinarily successful.” 

But culture is not exactly unique to humans. Boyd and Richerson postulated that other species should also develop culture. “Given that all environments are spatially heterogeneous [all life faces a range of challenges] and all mammals, and plants for that matter, migrate,” they write, “culture should be more common, if not ubiquitous.”

Although many anthropologists once saw culture — the things animals learn from each other — as uniquely human, we’ve come to realize that many other animals do pass learned skills from one generation to the next. Even beyond direct learning, some can learn from individuals of different communities and can even follow trends. What makes human culture somewhat distinct is that we can record the knowledge we gain over multiple generations and learn indirectly through things like stories, books, and more recently, YouTube. This, in turn, has allowed us to make unbelievably complex tools.

It’s plausible that our large brain — and resulting processing power — gave us a unique capacity to develop human culture, which in turn allowed us to become successful. But it’s not entirely clear why primates, and ultimately Homo sapiens, developed such large brains in the first place.

There are several hypotheses on this point. One possibility is that primates developed large brains “to manage their unusually complex social systems,” psychological anthropologist Robin I.M. Dunbar explained in a 2009 Annals of Human Biology paper. Another possibility is that big, complex brains were necessary to survive in tough, constantly-changing environments. In reality, it’s probably a combination of both hypotheses, says Richard Potts, a paleoanthropologist at the Smithsonian Institute’s National Museum of Natural History. A challenging environment could spark the development of a brain that’s capable of complex culture and social interaction. As Boyd and Richerson put it: “Ancient mammals were dull because they lived in a dull, little-varying world, whereas modern mammals are sharp because they live in a world alive with rapid change.” 

Ours is a story of adaptability, says Potts. Environmentally speaking, “all hell broke loose around 400,000 years ago,” he explains. “There was not only vast changes in climate, but there was also — for whatever reason — a real increase in the amount of tectonic activity, in other words earthquake activity.” This broke up the large East African grasslands that hominids used into smaller regions dotted with lakes that appeared and disappeared within several tens of thousands of years — a relative blink of an eye in geologic time. The landscape was in a constant state of flux, and those species that survived the changes actually came out better off than they were before.

For animals exposed to fluctuations 400,000 to 300,000 years ago, the theory goes, ones with beneficial adaptations survived. During this time, our ancestors expanded their tool box with stone points and started long-distance trading networks — evidence of an increase in brain power, Potts says. 

But perhaps it was something more fundamental than the brain. Consider other species that have done well as spreaders and overtakers: invasive species. Some of the most prolific are those with much smaller brains than we have, and invasive plants have managed to spread quite far without any brains at all.

“They’ve been able to escape their evolutionary controls,” says Marc Cadotte, an ecologist at the University of Toronto who studies invasive species. Rather than succeeding by just out-thinking their competition or developing complex tools, invasive plants uprooted and moved — often with human help — to a location where they had an advantage, such as fewer predators.

Most anthropologists agree that around 60,000 years ago, Homo sapiens left East Africa. Although new evidence is constantly changing the timeline, the fact remains that early humans moved. While we don’t know what these early explorers were thinking, we do have some evidence suggesting their movement was intentional — perhaps a journey to find environmental stability, food, or maybe an escape from evolutionary controls.

“There’s something special about the human ability to look over the next hillside and into the next valley and to keep going,” Potts says. Scientists don’t have much evidence that other animals — even far-ranging species like saber-tooth cats — spread to new regions as we did, Potts says, so we seem to be unique. When animals encounter new environments, they must decide to stay or continue moving, he adds. We often choose to keep moving. 

As we moved, we continued to gain an advantage over local species. Invasives “are able to escape their natural controls — their predators, their pathogens, etcetera,” Cadotte reminds us, and as humans, “we are very good at doing that.” Modern humans develop medicines to escape microbes that would do us in, he explains, and we tend to decimate large animals wherever we go, which keeps us from getting eaten or having to compete for resources. So not only are we suited to be explorers, we are highly competitive invaders. 

And it’s not clear why we Homo sapiens survived and expanded when our close relatives — such as Neanderthals — did not. Volumes of studies have tried — only somewhat successfully — to answer that question. Perhaps we had those “small advantages,” that Darwin predicted could make a “big difference.” After all, recent research suggests Neanderthals may have suffered from poor diet, while ours may have been better. Or maybe we were just a bit more persistent in our efforts to migrate. Regardless, it’s clear that we have colonized the world unlike any other species.

As such, we cause a lot of problems. Our spread around the planet is linked to mass extinctions, says Dustin Penn, editor of the book “Evolutionary Perspectives on Environmental Problems.” When humans crossed the land bridge into North America about 15,000 years ago, for example, mass extinctions of mammoths, saber-tooth cats, and New World horses followed and were likely caused by humans, according to a 2018 Nature Communications study. “The problem with these correlations is that the spread of humans was also correlated with climate change, which could have, by itself, caused a mass extinction,” Penn says. So it’s hard to know exactly how much blame to place on early humans. Still, it’s likely that we played our part — sometimes a big part — in driving out the locals.

“In an evolutionary sense,” Penn says, “you might simply conclude: Humans have been extremely successful organisms.” Considering how far we’ve spread and how numerous we’ve become when compared to our humble origins, we are evolutionary winners — thanks to the challenges our ancestors faced. But, he adds, “there’s nothing to suggest that, by our being successful, we will have the wisdom to act in our long-term interest.”

About the Author

Curtis is a photographer and science journalist who focuses on health, Earth science, and ecology. Growing up in New Mexico, his life was centered around nature—hiking, biking, and exploring. When he wasn’t outdoors, he was reading (he loves travelogues). Later, while studying geology at Trinity University, he realized he could combine these passions by becoming a science journalist. Now, he uses his words and photos to help others see practical beauty in science.

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