The mouse looks the same but has an ancient gene
By engineering its genetic blueprint, they have rebuilt a gene that was present in primitive animals.
The ancient gene later mutated and split, giving rise to a pair of genes that play a key role in brain development in modern mammals.
The scientists say the experiments shed light on how evolution works and could lead to new gene therapy techniques.
"We are first to reconstruct an ancient gene," said co-researcher Petr Tvrdik of the University of Utah. "We have proven that from two specialised modern genes, we can reconstruct the ancient gene they split off from.
"It illuminates the mechanisms and processes that evolution uses, and tells us more about how Mother Nature engineers life."
The study, published in the academic journal Developmental Cell, involved a suite of genes involved in embryonic development.
Until about 500 million years ago, early animals had 13 such Hox genes. Then each gene split into four, making 52 genes.
Over the course of evolution, further mutations occurred, and some genes became redundant and disappeared, leading to today's tally in mammals of 39 Hox genes.
The Utah team looked at two of these genes; Hoxa1, which controls embryonic brain development, and Hoxb1, which plays a key role in the development of nerve cells that control facial expressions in animals.
The Utah pair combined critical sections of each gene, reconstructing a gene similar to its equivalent some 530 million years ago.
The hybrid gene is not completely identical to the ancient one, but the scientists say it performs essentially the same functions.
"What we have done is essentially go back in time to when Hox1 did what Hoxa1 and Hoxb1 do today," said Mario Capecchi, professor of human genetics at the University of Utah School of Medicine.
"It gives a real example of how evolution works because we can reverse it."