From the towering skyscrapers of modern metropolises, to the pyramids and other ancient buildings left behind by history, we can see traces of human past and present existence throughout the earth today. Vast farmland and extensive roads also mark the trails of our activity. But what would the world look like if humans never existed?
Scientists paint a picture of pristine wilderness and colorful species, both familiar and unfamiliar. Trevor Worthy, a paleontologist and associate professor at Flinders University in Australia, said: "I think the Earth will be a more lush place with a large number of animals, large animals scattered across all continents except Antarctica. ”
A world without modern humans may also mean that our extinct human relatives, such as Neanderthals, will still exist. And they, no doubt, will also change the landscape of the entire planet.
Humans have shaped the world at the expense of many species, from the Raphus cucullatus to the Tasmanian tiger (Thylacinus cynocephalus), and by hunting and destroying habitats, we have pushed these beautiful lives to the point of extinction.
According to the most conservative estimates, the extinction rate on Earth today is more than 100 times higher than it was when there were no humans, and there has never been such a high rate of extinction on Earth since the Cretaceous-Paleogene (K-Pg) extinction event, which wiped out about 80 percent of animal species, including non-avian dinosaurs. In other words, humans are like an asteroid, hitting the planet, and as dust still settles, wildlife continues to decline.
"My great-great-grandfather had seen flocks of long-tailed parrots in nature, my grandfather had seen a hundred, my father had seen some, and if I could see two in the forest, I would have been very lucky." Worthy said.
The human-led decline of nature suggests that without us, Earth would be a much wilder place, with some vanished megafauna, such as moa, standing out more than other behemoths. It's an ostrich-like bird, some as tall as 3.6 meters, that evolved in New Zealand for millions of years, but just 200 years after humans invaded their land 750 years ago, all nine species of moa, and at least 25 other vertebrate species, disappeared from the planet forever, including the moa-feeding Giant Eagle (Hieraaetus moorei).
Giant moa birds and haast eagles are the most recent examples of the disappearance of large animals, and their extinctions are certainly linked to human activities, such as unsustainable hunting, and the introduction of invasive species into new habitats, so they can also serve as a reference for humans interacting with large animals elsewhere.
The survival of large animals is crucial to speculating what a planet would be like without humans, as these beasts have had a significant impact on the earth's landscape.
Sören Faurby, senior lecturer in zoology at the University of Gothenburg in Sweden, believes that humans played the most critical role in the disappearance of many large mammals thousands of years ago. He led a 2015 study published in the journal Diversity and Distributions, which showed that without humans, the earth would largely resemble what is now the Serengeti grasslands, a vibrant ecosystem in Africa.
If so, the large animals found in the Serengeti today — including elephants, rhinos, and lions — will also live throughout Europe. For example, not only the African lion, but also the cave lion, a slightly larger species that was hunted in Europe until about 12,000 years ago. According to Fabi, the Americas will also remain home to elephant relatives and large bears, such as armadillo relatives as large as cars, known as the sculpted beasts, and the unique species of the earth sloth.
"In a world without humans, large mammals will be more diverse, and if you see greater diversity of large mammals, you tend to see a more open habitat," Fabi told Live Science.
Elephants and other large animals are less constrained and less hindered in finding food. "If you grow big enough, it might be easier to knock down a tree and eat the fresh leaves on it," Fabi said. Of course, if there are too many large mammals, the first thing is that the lush vegetation will tend to decrease, he adds.
Large animals such as elephants are known as megafauna. During the last ice age of the Pleistocene (2.6 million to 11,700 years ago), the world had a rich fauna, but with the end of the Ice Age, or in the thousands of years that followed, most animals went extinct. For example, according to a 2020 study in the Proceedings of the National Academy of Sciences, about 38 large animal species went extinct in North America at the end of the last ice age. For the past century, scientists have debated whether changes in the natural climate, or overhunting by humans, are the main causes of the decline in the number of these large animals.
A 2021 study published in the journal Nature concluded that climate change eventually wiped out the mammoths (Mammuthus primigenius) that survived at the end of the Pleistocene, as well as other large animals that inhabit the Arctic, because the vegetation they ate could no longer survive as the climate warmed and became wetter.
Humans did, however, hunt mammoths. Not only Fabby, but other scientists who share his belief that humans may have been a key factor in their extinction believe that mammoths had survived climate change before humans appeared, and that if it weren't for the extra pressure that humans put on them, they might well have survived to this day.
Christopher Doughty, an associate professor and ecosystem ecologist at Northern Arizona University, simulates how large animals, past and present, spread seeds and nutrients by eating and defecating. His work shows that the transport of elements such as phosphorus, calcium and magnesium, which are vital to life, has been reduced by more than 90 percent due to the extinction of large animals.
Doherty speculates that without humans, the elements would be more evenly distributed throughout the landscape, which means more fertile soil and leads to greater productivity of ecosystems. "If the elements in the ecosystem are more fragmented, the productivity will also be more fragmented," Doherty said.
Doherty said humans have brought elements together through practices such as farming and building fences, and these areas will become less fertile over time compared to wild systems. Higher fertility means plants can allocate resources to more fruits and flowers so the world will look more vibrant and feed more animals.
Climates can also be different, and while it's hard to say how humans and megafauna influenced climate change thousands of years ago, because the evidence has been obscured by time, it's much easier to judge how we affect Earth's climate today. Due to global warming caused by activities such as fossil fuel combustion, humans have raised the earth's average temperature by about 1 degree Celsius since the beginning of the 20th century. So the planet would at least be much cooler without us.
A 2016 study published in the journal Nature concluded that anthropogenic warming would delay the upcoming ice age by at least 100,000 years. However, even without human delays, an ice age would not have come in 50,000 years, so if we were gone, it would be unlikely that The Planet would be in another ice age.
Our modern humans today (Homo sapiens) are not always the only race in this landscape, and even removing us from this equation could open the door to our Neanderthal cousins. Scientists aren't sure why Neanderthals went extinct about 40,000 years ago, but because they crossed with Homo sapiens, part of their DNA exists in us. Neanderthal deaths may have had a variety of causes, but we are the prime suspects.
Chris Stringer, professor of human origins and head of research at the Natural History Museum in London, believes that competition for resources was a factor in the disappearance of Neanderthals. "If we hadn't shown up, if we hadn't been in Europe 45,000 or 50,000 years ago, I think they might still be here," he told Live Science.
According to Stringer, Neanderthals lived complex lives in Europe, similar to modern humans, but they struggled to cope with climate change, and in relatively small numbers and low genetic diversity. This is bad news for any species, as it is a sign of inbreeding and poor health. It's likely that Neanderthals were "already in trouble, and when modern humans got there as well, I think it could have left them completely marginalized," Stringer said.
But it may not be just Neanderthals who are crushing humans. Scientists are still studying at least one other human lineage: Denisovans, who lived at the same time as modern humans and Neanderthals. Denisovans seem to be genetically and physically closer to Neanderthals than modern humans, but differ from Neanderthals in their very large molars.
Humans most likely had hybrids with Denisovans because there is evidence that denisovan DNA exists in modern humans living in places like New Guinea in Oceania — a finding that suggests that denisovans once interacted in Southeast Asia with their ancestors who later settled further east, the study published in the journal Science in 2012. According to previous reports by Live Science, denisovans also dealt with Neanderthals in Siberia, where fossil remains of denisovan-Neanderthal hybrid descendants have been found.
These Denisovan interactions, as well as fossil evidence, suggest that they had a larger geographical range and covered a more diverse environment than Neanderthals, so it can be argued that they adapted to a wider range than Neanderthals. According to Stringer, DNA evidence also suggests that Denisovans may have greater genetic diversity than Neanderthals. "They may be more likely to survive than Neanderthals."
Neanderthals and Denisovans are important because if one or both of these lineages survive, they may end up on a similar path to Homo sapiens, switching from hunter-gatherer to developing agriculture at the end of the last ice age.
"If there's enough time, there's no reason why Neanderthals or Denisovans don't end up doing this," Stringer said, adding that through evolution, they may have overcome any potential intellectual deficits, though it's unclear if they have those flaws, so the world may not look like much different after all.
"Again, they're probably going to make all the mistakes we've made," Stringer said, so the Earth will still be heated, but it could be pushed by mysterious Neanderthals or Denisovans, who knows?
Compiled from: livescience
原标题：How would Earth be different if modern humans never existed?
By Patrick Pester