[考古] by ftdonnaf
A few years ago, Jarvis and his colleagues made the surprising discovery that when a songbird, parrot or hummingbird is producing its learned vocalization, a set of seven similar structures in the birds’ brains become active. The finding was unexpected because the three avian groups are only distantly related to one another. At the same time, they are closely related to other birds that are not vocal learners. Flycatchers, for example, belong to the same order as songbirds—Passeriformes—yet no flycatcher species tested so far displays the trait.
One possible explanation, says Jarvis, is that the three groups of vocal learning birds had a common ancestor that possessed the skill. “But this means there would have been multiple losses of the ability over time, a sort of mass extinction of vocal learning,” he says. Another hypothesis is that vocal learners evolved similar brain structures independently over the last 65 million years, much the same way that birds and bats separately evolved wings that turned out to be so much alike.
Discoveries about the human brain support this latter hypothesis. Scientists conducting imaging studies have found that when people speak, parts of their brains’ cerebrums that are similar to those of vocalizing songbirds, parrots and hummingbirds become active. They’ve also found that the same neural pathways are damaged in people who have lost the ability to speak due to injury or stroke. Jarvis now believes that vocal learning most likely developed independently in humans and the three bird groups (as well as in other learners whose brains have not been studied)—yet it arose from a preexisting brain system, probably shared by all vertebrates, that controls learning to move.
阅读13 Song bird 背景阅读 by jeffxu
Vocal learning, the substrate of human language, is a very rare trait. It is known to be present in only 6 groups of animals: 3 groups of birds (parrots, songbirds, and hummingbirds) and 3 groups of mammals (bats, cetaceans[whales/dolphins], and humans). All other groups of animals are thought to produce genetically innate vocalizations. To understand this concept, it is important to distinguish vocal learning from auditory learning. Auditory learning is the ability to make sound associations, such as a dog learning how to respond to the sound "sit". All vertebrates have auditory learning. Vocal learning is the ability to imitate sounds that you hear, such as a human or a parrot imitating the sound "sit". Currently only vocal learners have been found to have forebrain regions dedicated to vocal learning and production of these learned vocalizations. Vocal non-learners only have been found to have non-forebrain vocal regions responsible for the production of innate vocalizations.
The objective of this project, a collaboration between Dr. Jarvis’ lab at Duke University and Dr. Mello’s lab at The Rockefeller University, is to determine how the vocal learning behavioral trait and associated brain structures evolved. We utilize vocalizing-driven gene expression to identify vocal brain structures in vocal learning and vocal non-learning species. To date, we have used this approach in 3 vocal learners - songbirds, parrots, and hummingbirds. We have found that they each contain 7 very similar brain structures. If according to the current dominant hypothesis, vocal learning evolved independently in all 6 vocal learning groups within the past 65 million years, then the striking similarities in brain structures of at least the 3 avian groups suggest that there a strong epigenetic constraints on how vocal learning can evolve. We are now determining if this hypothesis is correct, or if there really was a common ancestor with vocal learning, and other groups lost them through evolution. Above, click on the group name to find out about the latest results.