Ancient human footprints preserved in the ground across the White Sands National Park in New Mexico are astonishingly old, scientists reported on Thursday, dating back about 23,000 years to the Ice Age.

The results, if they hold up to scrutiny, would rejuvenate the scientific debate about how humans first spread across the Americas, implying that they did so at a time when massive glaciers covered much of their path.

Researchers who have argued for such an early arrival hailed the new study as firm proof.

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For half a billion years or so, our ancestors sprouted tails. As fish, they used their tails to swim through the Cambrian seas. Much later, when they evolved into primates, their tails helped them stay balanced as they raced from branch to branch through Eocene jungles. But then, roughly 25 million years ago, the tails disappeared.

Charles Darwin first recognized this change in our ancient anatomy. But how and why it happened has remained a mystery.

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A second dose of Johnson & Johnson’s vaccine substantially increased its protection against Covid-19, the company announced Tuesday morning.

In a clinical trial, researchers found that two doses of the vaccine delivered 94 percent efficacy against mild to severe Covid-19 in the United States, up from 74 percent conferred with a single shot, the company reported. And two shots showed 100 percent efficacy against severe disease, although that estimate had a wide range of uncertainty.

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