Beyond that, Andersen noted, barefoot humans of the past were not running around on hot asphalt and other modern surfaces.
The findings, published June 26 in the journal Nature, are based on just over 100 adults from Kenya and the United States. Both groups included people who said they were barefoot more often than not, and people who wore shoes every day.
As expected, the barefoot crowd had thicker, harder calluses. Despite that, they showed no lack of sensitivity in the soles of their feet. In contrast, thick-soled shoes do compromise tactile sensitivity when you're walking, the researchers said.
It's not clear what the implication of that might be. But, Lieberman's team points out, when your perception of a walking surface is dulled, that can affect gait and balance. So it raises the question of whether thick-cushioned shoes can contribute to falls in people at risk.
Lieberman stressed, however, that it's simply a question. He said controlled studies would be needed to figure out the answer - for example, a trial that compares cushioned shoes to "minimal footwear" in older adults.
Minimal footwear refers to shoes with thinner, harder soles - like moccasins or sandals. According to Lieberman, they more closely approximate thick calluses, compared with cushiony soles.
In other tests, the researchers found that cushioned shoes lessen the impact of the heel striking the ground with each footstep, compared with walking barefoot or in thin-soled shoes. Thick calluses did not have that effect.
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Yet cushioned shoes sent more force up into the joints with each step.
"The load is basically delivered to the knees," Lieberman said.
Again, the consequences of that, if any, are unknown. But one question, Lieberman said, is whether modern footwear could be a contributing factor to knee arthritis.
According to Andersen, it's an interesting question - but it would be challenging to study the way footwear choices over decades could affect arthritis risk.
"People generally wear all kinds of different shoes," she said. "There are also many other factors that would affect arthritis risk."
Plus, Andersen added, many people simply find minimalist shoes uncomfortable. "Even if wearing them for 30 years lowered your risk of knee arthritis, that's 30 years of being uncomfortable," she noted.
As for calluses, Andersen said that if they are not causing problems and you're healthy, they can probably be left alone.
