The morphological characteristics of extinct relatives of the giant panda suggest that while the ancient giant panda was omnivorous 7 million years ago (mya), it only became herbivorous some 2-2. 4 mya with the emergence of A. microta.  Genome sequencing of the giant panda suggests that the dietary switch could have initiated from the loss of the sole T1R1/T1R3 umami taste receptor, resulting from two frameshift mutations within the T1R1 exons.  Umami taste corresponds to high levels of glutamate as found in meat and may have thus altered the food choice of the giant panda.  Although the pseudogenisation of the umami taste receptor in Ailuropoda coincides with the dietary switch to herbivory, it is likely a result of, and not the reason for, the dietary change.  The mutation time for the T1R1 gene in the giant panda is estimated to 4. 2 mya while fossil evidence indicates bamboo consumption in the giant panda species at least 7 mya, signifying that although complete herbivory occurred around 2 mya, the dietary switch was initiated prior to T1R1 loss-of-function.
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