In the absence of current humans, our extinct ancestors, such as the Neanderthals, may still be roaming the earth. And, of course, they would have had a profound impact on the landscape.

Throughout history, from the dodo (Raphus cucullatus) to the Tasmanian tiger (Thylacinus cynocephalus), humans have transformed the planet at the expense of many species, which we have driven to extinction via actions such as hunting and habitat destruction.

According to the most conservative estimates, the extinction rate on Earth today is more than 100 times higher than it would be if humans did not exist, and it has not been higher since the Cretaceous-Paleogene (K-Pg) extinction event, which wiped out approximately 80 percent of animal species, including the nonavian dinosaurs, 66 million years ago. In other words, people have struck the globe like an asteroid, and the dust is still settling as wildlife populations continue to drop at an alarming rate.

“My great, great grandfather was able to observe flocks of thousands of parakeets in the natural landscapes, my grandfather saw flocks of a hundred, my father saw a few and I’m lucky if I can see two in the forests,” Trevor Worthy, a paleontologist and associate professor at Flinders University in Australia. 

With the destruction of nature caused by humans, it appears that Earth would be a wilder world without us, with some lost giants, such as moas, standing out more prominently in our absence. During millions of years of evolution in New Zealand, this group of ostrich-like birds, some of which stood up to 11.8 feet (3.6 meters) tall, developed a distinctive appearance. According to Worthy, within 200 years of humans’ debut on these birds’ lands 750 years ago, all nine species of moa were extinct, as were at least 25 other vertebrate species, including the massive Haast’s eagles (Hieraaetus moorei) who hunted the moas.

Gigantic moas and bald eagle-like birds of prey such as Haast’s eagles are recent examples of huge creatures whose extinction can be traced back to human activities such as unsustainable hunting and the introduction of exotic species into new environments. Moreover, they provide clues as to what a human relationship with giant creatures would have been like in other parts of the world.

Because huge animals have such a significant impact on landscapes, the survival of these species is essential when speculating about a world without humans.

Modern-day Serengeti

Swedish zoology professor Sören Faurby believes that humans have played a significant role in the extinction of many huge mammals over thousands of years, and this has been true since the beginning of recorded history. The lead author of a 2015 study published in the journal Diversity and Distributions indicated that, without humans, Earth would be most similar to the modern-day Serengeti, an African ecosystem teeming with life.

In this scenario, extinct creatures similar to those seen in the Serengeti today — such as elephants, rhinos, and lions — would roam freely throughout Europe, including the United Kingdom. Consider the following scenario: In the absence of African lions (Panthera leo), there would still be cave lions (Pantheras spelaea), a somewhat larger species that thrived in Europe until approximately 12,000 years ago. Meanwhile, according to Faurby, the Americas would be home to elephant relatives and gigantic bears, as well as unique species such as car-sized armadillo relatives known as Glyptodon and giant ground sloths, which are related to elephants.

“In a world without humans, there would be a much bigger diversity of large mammals, and if you see a larger diversity of large mammals, you tend to see a much more open habitat,” said Faurby. 

Elephants and other large animals are quite determined when it comes to getting food, and they will not tolerate any needless obstacles in their path.

According to Faurby, “if you’re big enough, then it might be easier to just knock over a tree and eat the fresh leaves on top.”

But, he continued, if there are a big number of large mammals there, there is a greater likelihood of less forested vegetation forming in the first place.

The woolly mammoths in the room

Megafauna were large animals that include elephants and other similar species. Although megafauna were abundant throughout the last ice age of the Pleistocene (2.6 million to 11,700 years ago), most of them died out as the ice age ended or in the millennia that followed. According to a study published in the journal Proceedings of the National Academy of Sciences in 2020, around 38 genera of large animals went extinct in North America during the end of the last ice age, including wolves and bears. Scientists have been debating whether natural climate change or human activity, such as overhunting, are the primary causes of the extinction of these enormous creatures for more than a hundred years.

In a 2021 study published in the journal Nature, researchers concluded that climate change was ultimately responsible for the extinction of woolly mammoths (Mammuthus primigenius) and other Arctic-dwelling megafauna that survived the end of the Pleistocene, because the warming climate made it too wet for the vegetation they consumed to survive.

Mammoths, on the other hand, were hunted by humans. Scientists who believe humans were the primary cause of their extinction, such as Faurby, say that mammoths survived climate change before humans arrived and would have likely lasted to the present day if not for the added strain humans put on them.

Northern Arizona University’s Christopher Doughty, an associate professor and ecosystem ecologist, simulates how huge animals in the past and present move seeds and nutrients around by eating and defecating. According to his findings, the transfer of essential minerals such as phosphorus, calcium, and magnesium has decreased by more than 90% as a result of the extinction of large animals.

Doughty believes that if humans were not around, elements would be spread more equitably across the landscape. This would result in more fertile soil, resulting in more productive ecosystems.

“If the elements are more patchy in ecosystems, the productivity is going to be more patchy,” Doughty said. 

According to Doughty, humans tend to clump components together through practices like agriculture and the development of fenced-off zones, making these areas less fruitful over time than wild systems. Greater fertility allows plants to focus their efforts on producing more fruits and flowers, resulting in a more vibrant world that feeds more animals. 

While it’s impossible to say how people and megafauna may have affected climatic shifts thousands of years ago since evidence has been clouded by time, judging our impact on Earth’s climate today is considerably easier. Since the beginning of the twentieth century, humanity has increased the average world temperature by around 1.8 degrees Fahrenheit (1 degree Celsius) due to global warming-induced by activities such as the burning of fossil fuels. As a result, without humanity, Earth would have been at least that much cooler.

According to a 2016 study published in Nature, human-caused warming will push out the next ice age by at least 100,000 years. Even without the human delay, it wasn’t due for another 50,000 years, so it’s doubtful that Earth would be in the midst of another ice age today if we weren’t here.

Humanity is unavoidable

Modern humans (Homo sapiens) were not always the only hominins on the block, and removing us from the equation may have allowed our Neanderthal cousins to enter. Scientists aren’t clear why Neanderthals became extinct some 40,000 years ago, but because they interbred with H. sapiens, some of their DNA is still present in some of us. There were most likely several causes for the extinction of the Neanderthals, but we are the primary candidate.

Chris Stringer, a professor and research leader in human origins at the Natural History Museum in London, believes that competition for resources had a role in the extinction of the Neanderthals.

“If we hadn’t been around, if we hadn’t come into Europe 45,000 or 50,000 years ago, I think they probably would still be here,” he said. 

Stringer claims that Neanderthals lived complicated lifestyles in Europe, similar to modern humans, but they struggled to adapt to climate change and were very few in number, with minimal genetic variety. This is bad news for any species since it indicates inbreeding and poor health. Stringer believes that Neanderthals were “already in trouble, and when modern humans got there as well, I think that may have been what tipped them over the edge.” 

Humans may have held back on more than simply Neanderthals. Scientists are still learning about the Denisovans, another human lineage that lived about the same time as modern humans and Neanderthals. In terms of DNA and appearance, this branch looks to be more similar to Neanderthals than contemporary humans, yet it can be distinguished from Neanderthals by its unusually large molars.

According to a 2012 study published in the journal Science, humans likely interbred with Denisovans because there is evidence of Denisovan DNA in modern-day humans living in places like New Guinea in Oceania — a finding that indicates Denisovans were in Southeast Asia interacting with the ancestors of modern humans who later settled further east. In Siberia, where the petrified bones of a Denisovan-Neanderthal hybrid were discovered, Denisovans collaborated with Neanderthals.

These Denisovan exchanges, together with fossil data, imply that they had a bigger geographic range than Neanderthals, embracing a wider range of settings, and were thus arguably more widely adapted. According to Stringer, DNA data reveals that the Denisovans had more genetic variety than the Neanderthals.

“They might have been an even better bet for survival than the Neanderthals.” 

Neanderthals and Denisovans are important because if one or both of these lineages had survived, they could have followed in the footsteps of H. sapiens, transitioning from hunter-gathering to developing agriculture when the last ice age ended.

“There’s no reason why Neanderthals or Denisovans couldn’t have done that eventually, given enough time,” Stringer said.

He noted that evolution may have helped them overcome any potential intellectual flaws, which he doesn’t believe they possessed. After all, the globe might not be so different after all.

“And equally, they could be making all the same mistakes we’ve made ever since,” Stringer told Live Sciences.

“So, global warming could have still been here, but with Neanderthals or Denisovans driving it, not us. Who knows?”

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