Evolution

Panda Evolution



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The giant panda (Ailurpoda melanoleuca) is a bear.  But it is a vegetarian bear.  Bears are omnivorous but will often prefer meat or insects in order to help them put on enough fat for winter, when they hibernate.  But giant pandas do not hibernate as well as not eating the usual diet of other bears. 


For many years, this has puzzled biologists to the point where giant pandas were classified as giant raccoons.  But genetic testing proved they were bears.  How could bears have become vegetarians?  Through natural selection over hundreds of thousands of years.


Fossil Pandas


The oldest panda fossils are about over 8,000,000 years ago known as Ailurpoda lufengensesis.  Fossils have only been found in China, but the species may have already been sticking to bamboo forests.  This ancestor was much smaller than the resent day panda.  We do not know what it ate.


The next major change happened 3 million years ago with the rise of Ailurpoda microta, which eventually were replaced 600,000 years ago by Ailurpoda melanoleuca baconi.  They were mostly found in what are now China, Taiwan and Vietnam.  These pandas were much larger than modern pandas, perhaps because they had easier access to food.  Just exactly what they ate is also unknown.


The modern giant panda bear is a very old species and does not differ much from Ailurpoda melanoleuca baconi. It may have lived as recently as 5,000 years ago.


Why Bamboo?


Bamboo comes in many species, grows rapidly and is (or was) a widely available food source.  Any species that could evolve to exploit bamboo did not have to worry about finding enough food.  Contrast this to other species of bears, which has to tend with finding food with enough calories in order to get them through hibernation.  Giant pandas can eat some other foods like fruit, dead fish and honey, but they evolved to chomp down bamboo for most of their waking hours.  When given a choice, they often prefer bamboo shoots, stems and leaves.


The ancestors of giant pandas found that they could survive on bamboo. Although nutritionally poor, pandas then didn’t have to waste energy hunting, fighting with others for carcasses or storing enough fat to last the winter.  This evolutionary strategy worked for thousands of years.  Until man came along, there was more than enough bamboo species growing to fill the bellies of all healthy pandas.


Ancient pandas probably did not have such a bamboo-heavy diet.  But there may have been heavy competition for all of these other foods, except bamboo.  Over thousands of years, the panda developed a taste for bamboo over all other foods.  This strategy worked well when there wasn’t much competition for bamboo.  But now humans have decimated the bamboo forests in a few hundred years.  This is far too short a time for the panda’s digestive system to adapt to another diet.  And so they hover at the brink of extinction.


The Thumb


Another adaptation pandas have made to their bamboo-heavy diet is the knob they have on their paws that does not appear in other bear species, although it does appear in the red panda (not a bear.)  Although not a fully fledged digit, is does have a core of bone, like a thumb.  It’s perfectly shaped to hold bamboo shoots.  It is unknown how many thousands of years the pandas had this thumb.  It certainly didn’t pop up overnight in one generation.


Perhaps pandas that could manipulate bamboo shoots in their mouths were able to breed more successfully than those that could not.  Over thousands of generations, pandas that could hold onto shoots bred more than pandas that could not hold onto shoots.  And so the panda’s thumb became larger and more noticeable.


References:


“The Blind Watchmaker – 10th Anniversary Edition.” Richard Dawkins. Norton and Company. 1996.


Athro, Limited: “The Panda’s Thumb.” http://www.athro.com/evo/pthumb.html


BBC h2g2. “Giant Pandas.” http://www.bbc.co.uk/dna/h2g2/A33105494


GrizzlyBear.org. “The Panda Bear.” http://www.grizzlybear.org/bearbook/panda_bear.htm


Summary: http://www.pandaworld.cn/englishsite/expertDetail.asp?id=518

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