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"Either the placode was ancestral for everyone and then it was lost multiple times in independent lineages of reptile … or birds and mammals invented placodes independently," he said.

The second possibility seemed particularly unlikely because research had revealed that the same exact gene, called EDA, controlled placode development in both groups.

By analyzing the genomes of the naked bearded dragons, they narrowed down the culprit to a mutation in a single gene, called EDA, which is also known to disrupt placode development in birds and mammals.

The experiment suggests that EphB2 tells the placode which way is up.