These tools have the capability to allow us to start to identify the many small molecular building blocks that make us unique as a species," Reilly said.

James Xue of the Broad Institute is lead author of the study.

The researchers used a technology called Massively Parallel Reporter Assays (MPRA), which can simultaneously screen and measure the function of thousands of genetic changes among species.

[Such deletions] can tweak the meaning of the instructions of how to make a human slightly, helping explain our bigger brains and complex cognition," he said.

The deletion of this genetic information, Reilly said, had an effect that was the equivalent of removing three characters -- "n't" -- from the word "isn't" to create a new word, "is."

In their study, the Yale team found that some genetic sequences found in the genomes of most other mammal species, from mice to whales, vanished in humans. But rather than disrupt human biology, they say, some of these deletions created new genetic encodings that eliminated elements that would normally turn genes off.

The paper was one of several published in Science from the Zoonomia Project, an international research collaboration that is cataloging the diversity in mammalian genomes by comparing DNA sequences from 240 species of mammals that exist today.

Often we think new biological functions must require new pieces of DNA, but this work shows us that deleting genetic code can result in profound consequences for traits make us unique as a species," said Steven Reilly, an assistant professor of genetics at Yale School of Medicine and senior author of the paper.

The fact that these genetic deletions became conserved in all humans, the authors say, attests to their evolutionary importance, suggesting that they conferred some biological advantage.

These 10,000 missing pieces of DNA -- which are present in the genomes of other mammals -- are common to all humans, the Yale team found.