Ancient Forest Shift Led to Primate Ancestors: Asteroid Impact and Seed Evolution

Scientists discovered evidence linking a sudden ecological shift in ancient forests to the rise of early primate ancestors. The catalyst was likely an asteroid strike approximately 65 million years ago, which decimated 50% of plant and animal life. Among the immediate effects of this event was the elimination of large sauropod dinosaurs. These dinosaurs had previously shaped the forest by trampling the understory, knocking down trees, and consuming large quantities of vegetation. With their disappearance, the forest became denser, leading to the development of precursors to many foods consumed today. "Seed size had been pretty stable, and then as soon as you had the dinosaurs go extinct, the seed size increased by orders of magnitude," stated Christopher Doughty, a professor at Northern Arizona University. Larger seeds provided an advantage in the darkened forest, enabling trees to grow taller and faster to reach sunlight. These trees produced large fruit to attract animals for seed dispersal, as survival under the mother tree's canopy was unlikely. "Seed dispersal syndrome wasn't very common. It took the resetting of the ecology for it to really develop," Doughty explained. Over time, these seeds and fruit became a crucial food source for early primate ancestors. Doughty, an expert in ecoinformatics, specializes in understanding how animal movement influences ecosystems. His team's research supports the theory that the proliferation of large seeds and fruit aided the flourishing of pan-primates like Purgatorius [pur-guh-TOR-ee-us] and Plesiadapiformes [plee-zee-ad-uh-PIE-for-meez]. They developed a computer model based on analyses of how large animal movement affects plant growth. "I have a PhD student who looked at how forest elephants opened up the understory in tropical forests. Megafauna aren't quite sauropods but they're big and we have data on how they impact forest structure," Doughty noted. The team compared their model with observed trends in seed and animal size, finding a close correlation over time. Examining the past 35 million years, they also observed a reversal in seed growth trajectory after land mammals grew larger and the forest opened up again. The model also accurately predicted that seed size would increase again after the extinction of prehistoric animals like mammoths approximately 50,000 years ago. The researchers are now using the model to understand the future impact of current mass extinction events caused by humans. They are examining how the loss of animals like rhinoceroses, elephants, and giraffes could affect ecosystems. "We've got relics of them, but we've almost got rid of all the big animals. In South America for instance, the biggest animal right now is the tapir [TAY-per], but there were something like 40 mammals larger than it that co-evolved with it and overlapped with the first humans," Doughty said. Human selective logging of unprotected trees is currently having the biggest impact on forests. Doughty compares this impact to the work once done by sauropods, but understanding the long-term effects is challenging. "When we try to make a prediction on the future of seed size you can't really, because we don't know what our own future will be," he concluded. "We could last millions of years, opening up the understory with selective logging. That would impact seed evolution a different way to if we went extinct. But either way the forest could remain dark because we might have caused the extinction of large mammals. There are alternative trajectories of potential seed size that are fun to think about."