Exploring the Human Impact on Evolution and Mass Extinction

Many factors drive the unrelenting world of natural selection, but it’s the human impact on evolution that’s become the greatest force of all.

Our effects on the environment are common knowledge, with global warming being one of the most talked-about subjects among politicians and environmentalists the world over. However, the human impact on the environment is just the beginning: We’re also changing the very path of evolution to such an extent that humanity is now one of the driving forces behind natural selection.

For the overwhelming majority of the last 200,000 years that modern humans have spent on Earth, we’ve been little more than small-time predators; hunter-gatherers that were, for a long time, incapable of even lighting a fire. We were also small in number, possibly reaching a global population of no more than 2 million by the beginning of the Holocene Epoch.

Today, there are 7.5 billion of us. Over a third of all potentially fertile land on the planet has been given over to agriculture, and we’ve presided over a 43% increase in atmospheric CO2 levels since the dawn of the Industrial Revolution. Added to these alarming statistics, we’ve also got everything from firearms to antibiotics to pesticides in our arsenal, jumpstarting an evolutionary arms race that few other species can keep up with.

How Mass Extinctions Shape the Course of Evolution

Donald E. Davis

Mass extinction events like the one that wiped out the non-avian dinosaurs, profoundly alter the course of evolution.

Almost all plant and animal genera that have ever lived are now extinct. Some estimates claim that as many as 4 billion species have lived on Earth at some point in the geological past. Today, some 8.7 million species exist in the eukaryotic domain, which includes all life other than bacteria and archaea. Of course, we can hardly blame ourselves for that – mass extinctions have claimed countless species for as long as life has existed on Earth.

Extinction plays a vital role in evolution to the extent that it’s the driving force between the success (or failure) of any species that has lived, does live and ever will live. Sometimes, it only takes a very minor catalyst to irrevocably change the course of Earth’s history. For example, had the Chicxulub impactor, which is widely believed to have been responsible for the extinction of the non-avian dinosaurs, had just a very slightly different trajectory, it would have safely flown right past the planet.

Thus, the Cretaceous-Palaeogene mass extinction wouldn’t have occurred, the dinosaurs wouldn’t have died, and mammals wouldn’t have risen to dominance in the ecological niches left behind by their disappearance. Likewise, humans wouldn’t exist either. It is, of course, impossible to tell beyond the wildest speculation what would have happened had it not been for mass extinctions such as that which defines the K-T boundary, but one thing is for certain – the world would be an unimaginably different place to what we know today.

Mass extinctions occur for many reasons, some of which are completely beyond the influence of biological processes. Volcanism, asteroid impacts and even a near-Earth supernovae and consequent gamma ray bursts have all been credited with mass dying and destruction.

In other cases, the success of a species may outpace all other natural influences. Perhaps the best example of all was the Oxygen Catastrophe of 2.3 billion years ago, when photosynthesising cyanobacteria became so prolific that it transformed the Earth’s atmosphere, making it inhospitable to anaerobic organisms.

Hunting and Habitat Loss of the Holocene Extinction

Mauricio Antón

The sudden disappearance of iconic Pleistocene megafauna coincided with the global proliferation of human hunting around 11,000 years ago.

There have been five major extinction events during the Phanerozoic Aeon, which spans the last 541 million years. Right now, however, we are living amid a sixth mass extinction, that which we might also call the Anthropocene Extinction, owing to our central role in its continuation. Global extinction rates are now estimated, according to some sources, to be over 1,000 times higher than the natural expected background rate. At this rate, almost a third of all plant and animal species will be gone forever by the middle of the twenty-first century, and we’ll largely have ourselves to blame.

At the time of writing, the IUCN Red List listed 5,210 species as critically endangered, 7,781 as endangered and 68 extinct in the wild. These alarming numbers continue to grow, offset only slightly by the fact that many of them will no doubt be joining the annals of history over the coming years. From the white rhino to the Sumatran tiger, the Earth is close to losing yet more of its most iconic megafauna.

Contrary to popular belief, the human-caused extinctions are nothing new. In fact, humanity has been the global super-predator for thousands of years. For example, the disappearance of much of the world’s megafauna abruptly following the end of the Pleistocene epoch 11,700 years ago, may also be due in part to overhunting by humans. In the Americas, for example, giant ground sloths, glyptodonts, smilodons and giant beavers all disappeared at around this time. As it so happens, it was about this time that people started spreading throughout those continents.

Much more recently, and throughout recorded history, there have been many more high-profile extinctions at the hands of humans. Some examples include the moas of New Zealand around the early fifteenth century and the overhunting of aurochs in the early seventeenth century.

In some cases, however, nature seems to be fighting back against the effects of hunting and habitat loss. Easily one of the most striking examples of the human impact on evolution in recent times is the fact that African elephants are starting to lose their tusks, apparently due to poaching.

Adding the Effects of Global Warming

NASA, NOAA

This heat map shows average temperatures have changed since 1880. Orange represents areas warmer than the baseline average between 1951 and 1980.

Until modern times, human-driven extinctions were caused by two things: habitat destruction and over-hunting. Today, however, we have climate change to contend with as well.

Before mankind started leaving its mark, CO2 levels largely increased because of volcanic activity, as appears to have been the case during the Cambrian Period, when they reached as much as 7,000 ppm. Just for comparison, that’s some 17.5 times greater than it is today! The world was also much warmer then, but the effects of atmospheric CO2 were partly mitigated by the fact that the Sun was cooler and ocean currents were completely different.

Methane is another potent greenhouse gas. It’s also one of the main by-products of life. It’s probably no coincidence, for example, that the world was a much warmer place at the time of the dinosaurs. Combined with higher CO2 levels, the flatulence of enormous sauropods and the like may have also played a key role in warming the planet. Perhaps, then, the dinosaurs were already on a path to self-destruction, had they not been short-circuited by a wayward asteroid.

The main drivers for global warming today are increased levels of CO2 and methane. The burning of fossil fuels, particularly coal, is largely responsible for manmade CO2 emissions, while reliance on livestock has increased methane emissions substantially. Of the two, however, CO2 emissions are by far the most prolific, and they’ve been responsible for bringing CO2 levels higher than they’ve been in hundreds of thousands of years.

With the exception of sudden catastrophic events, such as the impact that killed the dinosaurs, things like climate change normally take hundreds of thousands, if not millions, of years to occur. We’ve only needed a couple of centuries to do the same. As a result, we’re forcing animals and plants alike to adapt at an unprecedented rate, particularly when it comes to bird migration patterns and mating seasons.

Right now, we’re living in an interglacial period that we call the Holocene Epoch, which is part of the Quaternary Ice Age. These periods of glaciation are driven by the Milankovitch cycles, which are in turn controlled by the movements of the Earth around the Sun. However, our impact on the environment appears to be even greater, meaning that we might have even staved off the next glacial period by as much as 100,000 years.

Present rates of global warming, manmade or otherwise, are bad news for many of the world’s species. Plants and animals alike must adapt faster than ever to keep up with changing temperatures and differing rainfall patterns. In some cases, species are adapting to compensate for the human impact on their environments. However, evolution is supposed to occur over a million years, and not within a handful of generations.

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In the harsh and unforgiving world that is evolution, species are faced with two choices: adapt or die. From global warming to relentless habitat encroachment, we are shaping the future of life on Earth in many different ways. But does this mean that nature will be able to keep up with us, and what can we do to help stem the tide of destruction that we continue to cause? Let me know what you think in the comments below.

Further Reading