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What makes a modern human?

Just 1.5 to 7 percent of human DNA is unique to modern humans, Tina Hesman Saey reported in “Most human DNA is not unique to us” (SN: 8/14/21, p. 7).

Reader Mark Jenike wondered how the new research squares with the well-known finding that we humans share more than 98 percent of our DNA with chimpanzees and bonobos. “Are these two statistics measuring different things?” Jenike asked.

That shared DNA was likely passed down from the most recent common ancestor of humans, chimpanzees and bonobos, Saey says, and is common among all modern and ancient humans. The question asked in the study, however, is how much of our DNA is found only in anatomically modern humans, and not in Neandertals, Denisovans or other extinct ancient human relatives, she says.

“It’s important to remember that the 1.5 to 7 percent of DNA unique to modern humans consists of small genetic changes, and every single person today has them,” Saey says. “Overall, humans’ DNA still closely resembles that of other apes such as chimpanzees and bonobos.”

Reader Chuck Almdale wondered why the phrase “uniquely human” was used in the story to exclude Neandertals and Denisovans. If the ancient hominids could interbreed and have fertile offspring with modern humans, wouldn’t that make them the same species as us?

Drawing the line between species can sometimes be more of an art than a science, Saey says. Modern humans, Neandertals and Denisovans are genetically distinct groups, and while the groups were able to mate and reproduce with each other (SN: 5/8/21 & 5/22/21, p. 7), that doesn’t necessarily make them the same species, she says.

Even scientists don’t agree on whether these three groups really are separate species, Saey says. What’s more, not all scientists use the term “human” the same way.

Martian rumblings

Analyses of seismic waves picked up by NASA’s InSight lander suggest that Mars’ core is at least partially liquid and somewhat larger than expected, Sid Perkins reported in “Quakes offer a peek inside Mars” (SN: 8/14/21, p. 9).

Perkins wrote that volcanic activity in a region of Mars that’s home to large volcanoes could trigger quakes or seismic waves. Reader Jim Sanborn pointed out that this statement seems to contradict another story in the same issue, “Mars lakes appear to be mirages” (SN: 8/14/21, p. 8). In that story, Adam Mann reported that it’s unlikely that liquid lakes lurk under the Red Planet’s southern polar ice cap. An underground magma pool would be needed to keep the lakes from freezing, but the planet appears to lack recent volcanic activity, he reported.

It’s true that there haven’t been signs of ongoing volcanic activity on Mars in recent times, Perkins says, although a study published in the Sept. 1 Icarus suggests that lava may have flowed from a fissure in the planet as recently as about 50,000 years ago. If any molten rock were to move within the planet, either underneath ancient volcanoes or elsewhere, it could potentially trigger marsquakes and seismic waves, he says.

Corrections

“How muscle cells keep otters warm” (SN: 8/14/21, p. 13) incorrectly described the effect of surface area on heat loss. Small bodies with less surface area don’t lose heat faster; small bodies with greater surface area relative to their volume do.

“Roads to the good life” (SN: 9/11/21, p. 24) incorrectly stated the name of a journal in which research by psychologist Shigehiro Oishi of the University of Virginia and social psychologist Erin Westgate of the University of Florida appears. The study, which suggests that major life events can contribute to higher psychological richness, was published in Psychological Review, not Psychological Science.

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