In 1982, the palaeontologist Dale Russell imagined what the dominant species on our planet would be like if the dinosaurs had not become extinct. The result was the dinosauroid, a bipedal and intelligent reptile that would have evolved from the Stenonychosaurus (formerly Troodon) to take the place that we non-existent humans would not have occupied. Although Russell’s very popular idea has been much debated and criticised, it was based on a hypothesis that has traditionally been accepted: if not for the catastrophe that wiped out the dinosaurs, mammals would not have been able to thrive and give rise to humans. But is this really the case? Do we owe our existence to the opportunism provided by a cosmic fluke, or is it more complicated than that?
66 million years ago, an asteroid the size of a small city fell near what is now the Yucatan peninsula, ending the reign of the dinosaurs and ushering in a new era in Earth’s history. But there is always a degree of uncertainty in the study of the Earth’s past through its remains, so scientists reach a consensus when they consider the evidence to be sufficient, although they do not completely rule out alternative hypotheses. For example, some experts argue that a massive episode of volcanism in what are now called the Deccan Traps in present-day India played a major or minor role in the extinction of the dinosaurs.
From the decline of reptiles to the proliferation of the mammals
Either way, it is estimated that the great extinction that marked the boundary between the Mesozoic and Cenozoic eras wiped out not only the large non-avian dinosaurs—birds are dinosaurs too—but at least 75% of the species on the planet. In the new era that followed, the decline of reptiles gave way to the diversification and proliferation of another group of animals: the mammals. While this chronological sequence is undeniable, the cause-and-effect relationship is uncertain: was the rise of mammals due to the catastrophe that decimated the reptiles? What would have happened if the asteroid had missed colliding with the Earth?
The first factor to consider is the state of health of dinosaurs as a group at the time of the catastrophe. For decades, scientists have debated whether they were in their golden age or had begun an irreversible decline. The latter hypothesis is supported by the discovery of fewer species in the fossil record at the end of the Cretaceous, the last period of the Mesozoic era. Research by Fabien Condamine from the University of Montpellier suggests that “the number of species was in steep decline from 10 million years before the asteroid strike until the dinosaurs were wiped out,” due to global climate cooling and a lack of adaptation to it.
But there is still no consensus on this question. According to Kyle Atkins-Weltman and Eric Snively of Oklahoma State University, the lower number of species at the end of the Cretaceous that has inspired this hypothesis could be “the result of sampling and preservation biases”; these researchers have shown that dinosaur diversity then was higher than previously thought due to errors in fossil identification that have confused several species as one, and that these dinosaurs “were doing quite well for themselves before the asteroid ruined everything.”
A second question is whether mammals were ready to make the leap, and whether they would have made it regardless, asteroid or no asteroid. The truth is that these animals were by no means spared from the fiery catastrophe. According to Nicholas Longrich, a palaeontologist and evolutionary biologist at the University of Bath, around 90% of mammal species were also wiped out. So the subsequent success of these animals cannot be attributed to the fact that they emerged unscathed from the great extinction. But for mammals, says Longrich, “the recovery was remarkably rapid”: “Within 300 000 years, local diversity recovered and regional diversity rose to twice Cretaceous levels.” Mammals began to proliferate in number and diversity.
The right combination of opportunity and luck
For Longrich, this prodigious recovery of the mammals could only have happened because the reptiles disappeared, leaving the ecological niches they once filled empty. Therefore, according to this expert, if the asteroid had missed our planet on that day in the Cretaceous, mammals probably wouldn’t have been able to thrive long enough for us to be here today. This is not to say that mammals had nothing to do with their own success: recent research shows that this group was already more diverse before the impact than previously thought. Placental mammals—the ancestors of groups such as primates, lagomorphs (hares and rabbits) and carnivores (such as dogs and cats)—coexisted with dinosaurs at the end of the Cretaceous period, so the groundwork for success was already laid for those who managed to survive the great cataclysm.
As for the emergence of humans, and the question of whether reptiles would have taken our place had the great extinction not occurred, Longrich says, “it’s not impossible but it’s unlikely. The biology of an animal constrains the direction of its evolution. Your starting point limits your endpoints.” The scientist points out that although dinosaur brains increased in size over their long history, they were always very small: at the end of the Cretaceous, a monster like the Tyrannosaurus had a brain of 400 grams—ours is around 1.3 kilos—and that of the Velociraptor was a ridiculous 15 grams. By contrast, mammals began to develop large brains early on.
Indeed, this has been one of the most frequent criticisms of Russell’s dinosaur idea, that a species equivalent to us was inevitable, whether mammal or reptile. A widespread view among scientists is that it was only in Africa that a combination of factors drove human evolution. In Longrich’s words, “even with the dinosaurs gone, our evolution needed the right combination of opportunity and luck.” Without that great extinction, we might not be here today, but neither would there be anything like Russell’s dinosaur; humanoid reptiles remain the stuff of science fiction.
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