The Colorado River – which did not always flow through the Grand Canyon region – may have begun to carve its path through it after an ancient lake overflowed roughly 5.6 million years ago, say researchers. Their results suggest lake spillover was more important than processes like groundwater flow or erosion in informing the river’s modern path. The timing and mechanisms leading to the Colorado River’s integration into the Grand Canyon and its role in the canyon’s formation remain poorly understood. Geologic evidence indicates that the Colorado River existed in what is now western Colorado by about 11 million years ago; however, it did not exit the Grand Canyon on its modern course until about 5.6 million years ago, leaving a significant gap in the river’s early history. Previous studies have proposed that the river’s integration into its modern path involved a complex, multi-stage history of canyon carving and capture. One leading idea is that a series of closed basins once held large lakes that gradually filled and spilled over, eventually linking isolated drainage systems into a continuous river system reaching the ocean. Geological formations such as the Bidahochi Formation have been interpreted as evidence of such a paleolake system, supporting the spillover hypothesis, yet these conclusions remain contested.

Here, John He and colleagues used precise uranium-lead dating of zircon crystals from volcanic ash layers and sandstones to determine the age and origin of sediments within the Bidahochi Formation and other areas along the Colorado River’s course. These zircon age patterns act as a “fingerprint” to trace sediment sources and river connections over time. He et al. discovered that the composition and age distribution of zircon grains in the upper Bidahochi Formation closely match those found in known early Colorado River deposits. This similarity suggests a shared sediment source and indicates that the upper Bidahochi Formation was likely connected to the ancestral Colorado River by 6.6 million years ago. Other geological evidence, including increased sediment accumulation, strontium isotope ratios, and fossil fish assemblages, further indicate that Colorado River water was flowing into and gradually filling the basin for hundreds of thousands to over a million years before extending downstream. What’s more, the elevation and structure of ancient lake deposits indicate that a Colorado River-fed lake once rose high enough to overtop the Kaibab arch to spill over into the canyon region. According to the authors, while other mechanisms like groundwater flow or erosion may have played supporting roles, their findings suggest that lake spillover was the primary process establishing the Colorado River’s course through the Grand Canyon.

Large, warm-bodied fish, like sharks and tuna, may owe their dominance to being able to retain their own body heat, but that advantage comes at a cost. According to a new study, these mesothermic species require nearly four times more energy than other fish, and as oceans warm, their tendency to generate heat faster than they can lose it may push these already vulnerable species closer to the brink of extinction. A small fraction of fish species, like tuna and some sharks, have evolved the ability to retain metabolic heat within the body – a strategy known as mesothermy – which can enhance their physiological capabilities. However, while the advantages allow such species to dominate as top ocean predators, they also come with elevated energetic costs, as maintaining elevated body temperatures and high activity levels demands substantial energy. However, the energetics of warm-bodied mesotherms, which can heavily influence marine food webs, are poorly understood, particularly in rapidly warming ocean environments.

To address this gap, Nicholas Payne and colleagues developed a method to estimate routine metabolic rate (RMR) in fish by analyzing heat exchange in tagged individuals and combining the results with published respiratory data for the species. This allowed Payne et al. to assemble a comprehensive dataset spanning nearly the full spectrum of fish sizes – from microscopic larvae to massive 3-ton sharks – from a wide range of ocean temperatures for both ectotherms and mesotherms. Then, using this framework, the authors evaluated how body size, environmental temperature, and heat-retaining physiological adaptations shape energy demands. The findings show that mesothermic fish require nearly four times more energy than their cold-bodied counterparts, high energy costs that likely constrained body size and contributed to extinction risk in both living and extinct species. Moreover, the analysis revealed a scaling mismatch between heat production and heat loss, in which rates of heat production increase faster than heat loss as fish species grow larger, meaning larger mesothermic fish become increasingly warm-bodied. According to the authors, this creates an “overheating predicament,” which may explain why such species are more commonly found in cooler, deeper, or higher-latitude waters. However, as these cooler waters warm under climate change, large mesothermic fishes – many already vulnerable and under severe pressure from overfishing – face increasing energy demands and substantial overheating risk, elevating their threat of extinction.

In most narratives, the story of evolution is the story of organisms emerging from the ocean and eventually populating the land.

But for some species that evolution also involved a return trip. Dozens of major mammal and reptile groups ultimately made their way back to the beach and into the water. A new Yale study has undertaken the task of explaining when and how this happened — and which species fully re-committed to the life aquatic.