A preferred model of evolution concludes that it was incredibly unlikely for humanity to evolve on Earth, and that extraterrestrial intelligence is vanishingly rare.
But as experts on the entangled history of life and our planet, we propose that the coevolution of life and Earth’s surface environment can have unfolded in a way that makes the evolutionary origin of humanlike intelligence a more foreseeable or expected final result than generally thought.
The Hard-Steps Model
A few of the biggest evolutionary biologists of the twentieth century famously dismissed the prospect of humanlike intelligence beyond Earth.
This view, firmly rooted in biology, independently gained support from physics in 1983 with an influential publication by Brandon Carter, a theoretical physicist.
In 1983, Carter attempted to elucidate what he called a remarkable coincidence: the close approximation between the estimated lifespan of the sun—10 billion years—and the time Earth took to supply humans—5 billion years, rounding up.
He imagined three possibilities. In a single, intelligent life like humans generally arises in a short time on planets, geologically speaking—in perhaps tens of millions of years. In one other, it typically arises in in regards to the time it took on Earth. And within the last, he imagined that Earth was lucky—ordinarily it might take for much longer, say, trillions of years for such life to form.
Carter rejected the primary possibility because life on Earth took so for much longer than that. He rejected the second as an unlikely coincidence, since there is no such thing as a reason the processes that govern the Sun’s lifespan—nuclear fusion—should just occur to have the identical timescale as biological evolution.
So Carter landed on the third explanation: that humanlike life generally takes for much longer to arise than the time provided by the lifetime of a star.
To elucidate why humanlike life took so long to arise, Carter proposed that it must rely upon extremely unlikely evolutionary steps, and that the Earth is very lucky to have taken all of them.
He called these evolutionary steps “hard steps,” and so they had two predominant criteria. One, the hard steps should be required for human existence—meaning in the event that they had not happened, then humans wouldn’t be here. Two, the hard steps will need to have very low probabilities of occurring within the available time, meaning they sometimes require timescales approaching 10 billion years.
Do Hard Steps Exist?
The physicists Frank Tipler and John Barrow predicted that arduous steps will need to have happened just once within the history of life—a logic taken from evolutionary biology.
If an evolutionary innovation required for human existence was truly improbable within the available time, then it likely wouldn’t have happened greater than once, even though it will need to have happened a minimum of once, since we exist.
For instance, the origin of nucleated—or eukaryotic—cells is some of the popular hard steps scientists have proposed. Since humans are eukaryotes, humanity wouldn’t exist if the origin of eukaryotic cells had never happened.
On the universal tree of life, all eukaryotic life falls on exactly one branch. This means that eukaryotic cells originated just once, which is consistent with their origin being unlikely.
The opposite hottest hard-step candidates—the origin of life, oxygen-producing photosynthesis, multicellular animals, and humanlike intelligence—all share the identical pattern. They’re each constrained to a single branch on the tree of life.
Nevertheless, because the evolutionary biologist and paleontologist Geerat Vermeij argued, there are other ways to elucidate why these evolutionary events appear to have happened just once.
This pattern of apparently singular origins could arise from information loss resulting from extinction and the incompleteness of the fossil record. Perhaps these innovations each evolved greater than once, but just one example of every survived to the fashionable day. Perhaps the extinct examples never became fossilized, or paleontologists haven’t recognized them within the fossil record.
Or perhaps these innovations did occur just once, but because they might have happened just once. For instance, perhaps the primary evolutionary lineage to attain one among these innovations quickly outcompeted other similar organisms from other lineages for resources. Or perhaps the primary lineage modified the worldwide environment so dramatically that other lineages lost the chance to evolve the identical innovation. In other words, once the step occurred in a single lineage, the chemical or ecological conditions were modified enough that other lineages couldn’t develop in the identical way.
If these alternative mechanisms explain the distinctiveness of those proposed hard steps, then none of them would actually qualify as hard steps.
But when none of those steps were hard, then why didn’t humanlike intelligence evolve much sooner within the history of life?
Environmental Evolution
Geobiologists reconstructing the conditions of the traditional Earth can easily give you the reason why intelligent life didn’t evolve sooner in Earth history.
For instance, 90 percent of Earth’s history elapsed before the atmosphere had enough oxygen to support humans. Likewise, as much as 50 percent of Earth’s history elapsed before the atmosphere had enough oxygen to support modern eukaryotic cells.
The entire hard-step candidates have their very own environmental requirements. When the Earth formed, these requirements weren’t in place. As an alternative, they appeared in a while, as Earth’s surface environment modified.
We propose that because the Earth modified physically and chemically over time, its surface conditions allowed for a greater diversity of habitats for all times. And these changes operate on geologic timescales—billions of years—explaining why the proposed hard steps evolved after they did, and never much earlier.
On this view, humans originated after they did since the Earth became habitable to humans only relatively recently. Carter had not considered these points in 1983.
Moving Forward
But hard steps could still exist. How can scientists test whether or not they do?
Earth and life scientists could work together to find out when Earth’s surface environment first became supportive of every proposed hard step. Earth scientists could also forecast how for much longer Earth will stay habitable for different sorts of life related to each proposed hard step—corresponding to humans, animals, and eukaryotic cells.
Evolutionary biologists and paleontologists could higher constrain how repeatedly each hard-step candidate occurred. In the event that they did occur just once each, they might see whether this got here from their innate biological improbability or from environmental aspects.
Lastly, astronomers could use data from planets beyond the solar system to determine how common life-hosting planets are, and the way often these planets have hard-step candidates, corresponding to oxygen-producing photosynthesis and intelligent life.
If our view is correct, then the Earth and life have evolved together in a way that’s more typical of life-supporting planets—not within the rare and improbable way that the hard-steps model predicts. Humanlike intelligence would then be a more expected final result of Earth’s evolution, relatively than a cosmic fluke.
Researchers from a wide range of disciplines, from paleontologists and biologists to astronomers, can work together to learn more in regards to the probability of intelligent life evolving on Earth and elsewhere within the universe.
If the evolution of humanlike life was more probable than the hard-steps model predicts, then researchers usually tend to find evidence for extraterrestrial intelligence in the longer term.
