If E.T. arrives, they’re more likely to resemble birds than humans.
Humans are an outcome of mammal evolution.
Although mammals existed prior to the Cretaceous–Tertiary (CT) mass extinction, 66 million years ago, they were small, nocturnal, and restricted to narrow niches. Most mammal species disappeared during the CT extinction, along with all the non-avian dinosaurs. Geological profiles show a dividing line, with dinosaur fossils abundant below (before) the CT event layer, and mammal fossils abundant, and larger, above (after) it.
Biologists debate the degree of mammalian species diversification immediately before and after the CT event. Some paleontologists claim mammals explosively diversified (at 10 times ordinary rates) in less than half a million years. Genetic evidence renders this less likely. Instead, small mammals diversified into multiple groups, at low diversity, during the 20 million years leading up to the event. This is when placental mammals evolved, earlier mammals being marsupials and monotremes. Mammals that survived the CT event found the earth environment radically changed. The dominant Cretaceous species, dinosaurs, disappeared. Mammals evolved to fill the many niches now available, at around 3 times usual rates. New mammal species became much larger.
It took around 65 million years for hominids to evolve, once mammals took over earth. This may or may not be typical, in terms of the evolution of intelligence. As our only data point, I’ll assume it’s roughly ordinary.
Mammal ancestors first appeared 400 million years ago. All modern mammal females have milk-producing mammary glands, which defines the group. But these tissues do not survive in the fossil record, and probably weren’t present until shortly before or after the CT event. Other features have been used to detect mammal ancestors: teeth shapes, ear bones, palate bones, fur, and warm-bloodedness. Using palate bone evince, in particular, mammal’s earliest ancestors are posited to be the synapsids, a group separated from another vertebrate lineage, the sauropsids, between 320 and 315 million years ago.
252 million years ago the Permian–(PT) extinction event occurred. It was Earth’s most severe known extinction, with over half of all biological families vanishing. Although giant synapsids were important predators in the Permian period, the PT event upended everything. After 30 million years, an initially rare sauropsids group, the archosaurs, diversified into pterosaurs, crocodiles, and dinosaurs.
Dinosaurs dominated terrestrial ecosystems until the next great extinction, the CT event. During the dinosaur era, surviving synapsids probably evolved as small, nocturnal insectivores, in conditions that favored fur and higher metabolic rates, leading to warm-bloodedness. Warm-blooded animal species can maintain a body temperature higher than their environment. Currently only birds and mammals can do this.
During the roughly 300 million years prior to the CT event, the earth’s temperature was probably 4 to 8 C hotter than presently. That’s about the difference between the average temperature of Zurich, Switzerland (9.3 C) and Addis Ababa, Ethiopia (16 C.) Evidently the hotter, wetter Triassic climate did not require adaptive, high metabolisms necessary for warm-bloodedness.
Mammals succeed the dinosaurs because their ecosystem niches, at the time of the CT event, included those sufficiently protected to enable survival. Both survival and warm-bloodedness were consequences of being forced into protected places by dominant dinosaurs. Mammals then evolved a greater variety of forms in the first few million years after the dinosaurs went extinct than in the previous 160 million years of mammal evolution under dinosaurs.
Global temperatures remained high for around 15 million years after the CT event, then began to steadily decline. Warm-bloodedness became significantly more important over the past 35 million years, as periods of glaciation occurred, and overall temperatures creased. Warm-bloodedness may inhibit fungal infections. But more importantly, brain temperature regulation is critical for learning and memory.
Although some scientists suggest that dinosaurs may have been warm-blood, this is a decidedly minority view.
Consider, then, what might have happened without a CT extinction event. Dinosaurs would have continued their dominance, and mammals would have remained in their limiting niches. Their explosive evolution, leading to humans, would have been suppressed. But as temperatures fell, dinosaurs would have suffered as well. During glaciation periods, their territories would have shrunk, opening up large areas for competitors, including mammals. This would not afford the explosive radiation mammals exploited after the CT event, but a more modest evolutionary trajectory.
However another lineage, birds, would have evolved as they have. In fact, given their development of warm-bloodedness, birds would have exploited more environmental niches than any other group.
Primate evolution, a recent mammalian development, would have been delayed by many millions of years, perhaps 30 million or more. In the meantime, birds, which radiate more effectively, would have evolved at least as extensively as they have. The continued presence of dinosaurs would mean a larger genetic base to draw from, perhaps increasing bird diversity and development.
There are more species of birds than any other animal lineage, and more variety among them. Like mammals, some bird species have more intelligence than others. The crows, in particular, are analogous to primates, being the most intelligent.
Some crows perform mental skills as well as primates and dolphins. This may seem mundane, but in a world where mammals were relegated to the fringe, birds would have been major competitors. Instead of mammals growing ever larger, dominating the environment, birds would have dominated cooler regions.
If the evolution of highly intelligent mammals was delayed 30 million years, birds would have had ample opportunity to achieve symbolic intelligence first.
Since the CT event was random and unpredictable, any planet like earth cannot be expected to evolve intelligent mammals more quickly than intelligent birds. In fact, it’s more likely that the dominant lineage 100 million years ago, dinosaurs, would be the ancestor of the contemporary dominant species. With global cooling, birds would have become increasingly important. Eventually, species dominance would be determined by intelligence, leading to smarter and smarter birds.
Without a CT event, the evolution of intelligence could easily result in a bird, instead of human, capable of understanding how the universe works.
The Chicxulub meteor, that caused the CT extinction event, was a very random event. It hit part of the roughly 13 percent of earth’s surface that contains rocks that could burn off a tremendous amount of soot. Not coincidentally, the crater is in petroleum-rich geology. The soot cooled the planet by 8 or 9 C., even more over land. Dinosaurs suffered from lack of light, from the cold, starved, and were burnt in fires.
Had the asteroid hit elsewhere, no mass extinction might have occurred. Of course that would be true if no asteroid hit at all.
Other solar systems may have planets where life forms. Those planets may start hot and gradually cool, due to the physics of planetary formation. That may lead to life forms initially dominated by large, cold or cool-blooded types. As temperatures diminish and vary, flying species have an advantage in finding niches to dominate, and benefit from becoming warm-blooded. These would have the best chance of evolving higher intelligence, over sufficient time.
Furry, nocturnal insectivores, warm-blooded as well, would gradually emerge as significant competitors. But flyers related to the dominant species would have a head start, and may become ‘conscious’ first. Only a rare event that wiped out the dominant species would let other warm-blooded lineages take off. That may not usually happen.
This simple deductive exercise leads to the proposition that intelligent life on other planets, from other suns, will likely resemble birds just as much as they may look like humans. They’ll have roots in earlier dominant lineages. E.T. is probably more like a bird than a man.
