American Scientist

In this roundup, associate editor Nicholas Gerbis summarizes notable recent developments in scientific research, selected from reports compiled in the free electronic newsletter Sigma Xi SmartBrief.

The Secret of Birds’ Eyes

Scientists have cracked a key aspect of avian evolution: the mystery of how the keen eyes of birds can function despite their retinas being starved of oxygen. Birds lack the retinal blood vessels present in many other animals that supply vital oxygen to vision cells. With these treelike branches out of the way, more light reaches birds’ retinas without scattering, thereby sharpening their vision. But how do retinal cells function—and not die—without oxygen to fuel their energy production? Researchers at Aarhus University in Denmark say the answer is anaerobic glycolysis, a type of metabolism that generates energy by breaking down the sugar glucose in the absence of oxygen. This feat is remarkable because eyes (especially birds’ eyes) are quite energy hungry, and because glycolysis “burns” less efficiently than its oxygen-based counterpart, oxidative phosphorylation. That high-energy need should mean birds need more retinal blood vessels, not none at all. The solution comes from a long-debated structure called the pecten oculi, a comblike formation of blood vessels that juts into the eye’s interior. The structure pumps glucose into the vitreous humor (the clear gel filling the eyeball), which acts as a reservoir that feeds the inner retina, where it fuels anaerobic glycolysis. The pecten also helps remove lactic acid and carbon dioxide, metabolic waste products that can damage cells. The pecten oculi likely shares a common origin with a reptilian eye structure called the conus papillaris but developed after avians split from crocodiles.

Damsgaard, C., et al. 2026. Oxygen-free metabolism in the bird inner retina supported by the pecten. Nature. doi:10.1038/s41586-025-09978-w.

An Earlier Age of Fire

Scientists have found multiple compelling lines of evidence that humans made fire at least 400,000 years ago. That timing is 50,000 to 100,000 years earlier than previous, more equivocal evidence suggested, and 350,000 years earlier than the use of fire-making tools. The team, led by scientists at the British Museum, studied the Paleolithic site of East Farm Barnham in the Breckland of Suffolk, United Kingdom, where they analyzed a concentrated zone of clay-heavy silt that had been reddened by hematite, a mineral that can result from heating iron-rich sediments. Magnetic analysis showed repeated short-duration fires that suggested human campfires or hearths rather than wildfires. Also, ratios of chemical compounds formed when organic matter burns incompletely, called polycyclic aromatic hydrocarbons, were consistent with human fire activity. Spectroscopy revealed that the fires had reached temperatures in keeping with controlled fire use. Researchers also found two small fragments of pyrite that were oxidized and shattered, possibly through fire or through striking against flint. Significantly, pyrite can be used to create sparks for starting fires, but it is not common in the study area, which could mean an ancient hominin transported the mineral as part of a fire-making kit. Fire use marked one of the most transformative advances in human development, enabling greater security, better nutrition (which also supported larger brains), and greater social cohesion. Thus, the advent of fire-making suggests that a sea change in human behavior occurred around 300,000 to 500,000 years ago, which also corresponds with the development of near-modern brain sizes among modern human ancestors.

Davis, R., et al. 2026. Earliest evidence of making fire. Nature 649:631–637.

Armored Dinosaur Was a Baby

A guinea pig–sized dinosaur skeleton identified in Liaoning Province, China, 25 years ago belongs to an ankylosaur hatchling that had already begun to develop armor. Analysis of the bones of Liaoningosaurus paradoxus, which lived during the Early Cretaceous around 125 million years ago, refutes the hypothesis of a dwarf species of ankylosaurian and offers valuable insights into the development of these armored dinosaurs, some species of which reached 3 to 8 meters in length and weighed 500 to 4,000 kilograms. First described in 2001, the specimen baffled scientists because its features diverged so much from other ankylosaurs; for example, it had shell-like armor on its belly instead of its back, and pes (feet) double the size of its manus (hands). The argument hinged on the limitations of purely morphological evidence, which included several features that are more typical of young ankylosaurs than adults. But some purportedly juvenile characteristics—specifically an unfused gap separating two sections of vertebra—can also persist in fully grown Triassic marine reptiles. The bone histology, conducted by a team led by scientists at the Zhejiang Museum of Natural History in Hangzhou and the Chinese Academy of Sciences in Beijing, settled the argument. It revealed that the skeleton contains characteristics of young, fast-growing bone, including numerous canals for blood vessels, a barely formed medullary cavity (the interior hollow of long bones, which forms during maturation), and perhaps most tellingly the presence of a hatching line (a microscopic layer near the center of a bone that delineates the transition from prenatal to postnatal bone growth).

Zheng, W., Q. Zhao, P. M. Barrett, and X. Xu. 2025. Bone histology of Liaoningosaurus paradoxus (Ornithischia: Ankylosauria) from the Lower Cretaceous of Liaoning Province, China. Journal of Vertebrate Paleontology. doi10.1080/02724634.2025.2566325.