Scientists have diagnosed Down syndrome from DNA in the ancient bones of seven infants, one as old as 5,500 years. Their method, published in the journal Nature Communications, may help researchers learn more about how prehistoric societies treated people with Down syndrome and other rare conditions.

Down syndrome, which occurs in 1 in 700 babies today, is caused by an extra copy of chromosome 21. The extra chromosome makes extra proteins, which can cause a host of changes, including heart defects and learning disabilities.

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Naturalists have been trying for centuries to catalog all of the species on Earth, and the effort remains one of the great unfinished jobs in science. So far, researchers have named about 2.3 million species, but there are millions — perhaps even billions — left to be discovered.

As if this quest isn’t hard enough, biologists cannot agree on what a species is. A 2021 survey found that practicing biologists used 16 different approaches to categorizing species. Any two of the scientists picked at random were overwhelmingly likely to use different ones.

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Sixty-six million years ago, an asteroid slammed into the Gulf of Mexico. The catastrophe led to the extinction of as many as three-quarters of all species on Earth, including dinosaurs like Tyrannosaurus rex. But some flying feathered dinosaurs survived, and eventually evolved into the more than 10,000 species of birds living today, including hummingbirds, condors, parrots and owls.

Based on the fossil record, paleontologists have long argued that the asteroid’s impact was followed by a big pulse of bird evolution. The mass extinction of other animals may have eliminated a lot of competition for the birds, giving them the chance to evolve into the remarkable diversity of species that fly around us today.

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Women are much more likely than men to have their immune system turn against them, resulting in an array of so-called autoimmune diseases, like lupus and multiple sclerosis. A study published on Thursday offers an explanation rooted in the X chromosome.

The research, published in the journal Cell, suggests that a special set of molecules that act on the extra X chromosome carried by women can sometimes confuse the immune system.

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