But these poisonous sacs, known as parotoid glands, don’t just allow toads to wreak havoc as invasive species. They may also have helped the iconic amphibians, which originated in South America about 61 million years ago, conquer large parts of the planet. According to new research, which analyzed DNA from 124 species across six continents, early toads took an unexpected route: Instead of dispersing into Asia from North America via the Bering land bridge, as was previously theorized, they appear to have crossed directly from South America to Africa. Toads might have traveled through Antarctica, the study authors suggest, or sailed directly across the Atlantic Ocean on floating mats of vegetation.

The team also discovered an explosive rise in the number of new species shortly after toads began spreading out of South America. During that same period, toads evolved their parotoid glands, which ward off predators by secreting milky-white alkaloid substances called bufotoxins—an adaptation that likely helped this warty group of frogs rapidly colonize new habitats. “The parotoid gland,” study co-author Wei Xu explains, “was the real gamechanger.”

Researchers have designed a new wearable detector for the specific kind of longwave ultraviolet radiation that causes sunburns: UVA. The devices are fully transparent, allowing 75% of the sunlight to reach the semiconductor at their centers. That’s an advantage over previously developed, opaque sensors that allow less sunlight through and achieve less accurate measures of sunburn risk. The devices could connect with a smartphone or smartwatch to alert the user when they have received 80% of the UVA dose likely to cause a sunburn based on their skin type.

Though the solution may sound like a quick fix, it’s important to maintain usual sun protection like sunscreen or covering layers. But the authors noted that the technology “demonstrates its potential as a practical approach to prevent risks associated with prolonged UV exposure.”

The bold idea draws on evidence from lead found in fossil teeth from great apes and human ancestors such as Australopithecus africanus, Paranthropus robustus, the massive extinct ape Gigantopithecus blacki, ancient orangutans and baboons, Neanderthals, and early modern humans. “We expected some isolated findings, but to see consistent evidence across continents and species, from Australopithecus to Homo sapiens, was astonishing,” Renaud Joannes-Boyau, co-author on the new paper, told Science Adviser.

In modern humans, childhood exposure to the neurotoxic element—for example, through paint—has been linked to disordered emotional regulation and poorer impulse control and executive functioning. Such deficits might have set a sort of “ceiling” on just how complex ancient societies could become, the authors argue. “Lead exposure could have affected social behavior, communication, and even brain development in subtle but cumulative ways,” noted co-author Manish Arora.

To look for evidence of that, researchers turned to brain organoids—test tube minibrains engineered to have both modern and Neanderthal versions of a key brain gene called NOVA1. When they exposed these minibrains to lead in the lab, certain neurons in the Neanderthal organoids were severely damaged, while in the modern human organoids, these neurons remained essentially unharmed. “That was our eureka moment,” said lead author and neuroscientist Alysson Muotri.

The findings show that “environmental toxins aren’t just a modern challenge; they’ve been shaping biology and behavior for millions of years,” Arora added. “That’s an entirely new dimension to human evolution.”

The upshot, according to the team, is that our lineage must have evolved partial protection against the neurotoxic effects of lead at some point, clearing the way for humans to develop more cohesive societies that helped them weather climatic shifts and other existential threats. Neanderthals, by contrast, never developed these protections—and the ceiling on their social complexity may have been a contributing factor to their demise.

With contributions from Phie Jacobs, Hannah Richter, and Michael Price

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