Sloths might be the most misunderstood animals in the world, known mostly for their slow movement and tree-dwelling ways. Their interesting evolutionary history goes back 35 million years to the Late Eocene Epoch. Now, researchers have traced the captivating fossil nomenclatures of these ephemeral beings. Their goal was to learn from the success of 49 different lineages to understand their evolution before eventually going extinct. The results show just how terribly diverse sloths are. They previously included more than 100 different genera, ranging from miniature arboreal forms to colossal terrestrial forms.
Science has recently revealed just how acutely sloths diversified. They had changed for eons largely in isolation on the island continent of South America. Sloths have a fascinating diversity of form and function. Their earliest evolution is somewhat of a mystery since we don’t have fossil evidence that shows what they first looked like.
Ground sloths were common creatures across the continent, with outstanding representatives like Proscelidodon patrius mingling for thousands of years before their extinction circa 15,000 years ago. As people spread out across the Americas, especially in North America, sloth numbers started to decrease. This change suggests a more intricate interplay between the sloths and paleo Indian hunters. During excavations in the Pampas region of Argentina in the early 2000s, researchers discovered giant ground sloth bones. These bones had cut marks from stone tools, providing some of the earliest evidence for human interaction.
Scientific evidence demonstrates that humans were primarily responsible for the extinction of a wide array of megafauna—mammoths included—during the Late Pleistocene epoch. Sloths were rarely hunted. Juan Carrillo, a researcher involved in the study, noted, “Given the current evidence, there’s strong support for that.” He warned that we have to be a little careful. Mechanisms yet to be explored. He pointed out that more research is required to really understand these relationships.
A wide body size, then, became a matter of life and death for sloths. Animals that spent their lives in trees—like tree sloths—evolved along a starkly different trajectory than their terrestrial relatives. The larger sloth genera that roamed newly opened, open grasslands with their abundant, valuable food sources became more exposed and vulnerable to predation, leading to their eventual demise. As Greg McDonald pointed out, “Often we don’t get a chance to step back and get the big picture of what’s going on.” He observed that most published literature has been centered on individual findings or new species. Instead, it needs to be more forward looking, in exploring larger trends.
Although these gigantic relatives are long gone, modern sloths are more than represented by their two remaining tree-dwelling, slow-motion species. This grim climate shift from the formerly rich biodiversity exemplifies the profound transformation these creatures have endured over several million years.
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