Why do we need to eat protein if our bodies can create it?  Ribosomes
exist in both plants and animals, and they create proteins.  And the
proteins we do eat are broken down into their component parts which
means it does not matter if the protein source is plant or animal
or... from our selves!  Why can't our bodies make enough protein to
survive without eating it?

Five stars goes to an answer that references a site answering this
exact question that is not also selling me dietary supplements.

Five stars goes to an answer answers the question and that references
an animal that does not need to eat protein to survive.

Hello.

From an Ohio State chemistry lecutre:

" There are twenty amino acids that are found in proteins. Why do we
need to eat protein?
We need the amino acids that the body can not synthesize enough of."
http://class.fst.ohio-state.edu/biology101/lectures/L02.html

"We eat proteins to gain amino acids. There are 20 amino acids that we
must have in order to make proteins in our cells. We can make 11 of
those amino acids, and the amino acids that we can make are called
NONESSENTIAL AMINO ACIDS. The 9 amino acids that we cannot make are
ESSENTIAL AMINO ACIDS. It is important to consume "complete protein".
Protein from animals is usually complete, whereas protein from plants
is often lacking one or more essential amino acids. However one can
mix plant foods to get complete protein. For example, beans are low in
methionine but have enough tryptophan and lysine. Corn is low in
tryptophan and lysine, but adequate in methionine. Thus beans and corn
provide complete protein"
source: "Human Nutrition" Lecture
http://roberthamilton.pageout.net/user/www/r/o/roberthamilton/Lecture_13.html

"Proteins. We need proteins for the amino acids in them, which we use
to make other proteins.
1. 20 are essential, 11 of these we can make, and the other 9 we need
to get in our food
2. Complete proteins supply all requirements: milk, cheese, meat,
fish, poultry, eggs, soy
3. Incomplete proteins need to be balanced: for example, legumes and
nuts - balanced over one day
4. Excess protein over what is needed to make proteins is used for
energy or stored as fat
5. Meats often have fat as well
6. Recommendation is to have about 10% of calories from proteins."
source:
Wayne State: "Health Concepts"
http://www.is.wayne.edu/drbowen/gewf01/Agenda7.htm

-------

As for the last part of your question...

See...

"3. Cow stomach
Why is it that humans and most other animals need protein in their
diets, whereas animals like cows and sheep can live and thrive on a
diet of just hay and grass?
Dr. Sara Iverson, associate professor in the biology department at
Dalhousie University in Halifax, Nova Scotia."
CBC Radio
http://www.radio.cbc.ca/programs/quirks/archives/02-03/dec28.html#3

Listen to the MP3 sound file of the explanation (about 10 minutes or
so into the show)
http://www.radio.cbc.ca/programs/quirks/archives/02-03/mp3/qq281202a.mp3

Basically, Iverson explains that a cow can eat an essentially
protein-free diet because the bacteria in a cow's rumen ferment the
sugars from plants and grow microbes.  These ruminal microbes
synthesize their own amino acids that satisfy the cow's dietary amino
acid requirements.
This process is explained and illustrated in:
Chapter 1: Digestion in the Dairy Cow, from University of Wisconsin:
http://babcock.cals.wisc.edu/de/html/ch1/nutrition_eng_ch1.html

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search strategy:
"why do we need to eat protein"
"need proteins", "amino acids"
"animals need protein"

I hope this helps.

By my tally, you should get 10 stars.  Thank you - I'll have to ask
more questions here.

Thank you for the tip.
-juggler

The human genome is incomplete. It is composed of 22 pairs of
chromosomes, yet needs 24. Therefore, we do not manufacture enough of
the amino acids we need. We are in the process of evolving perfected
DNA structure. Thus it is that mankind becomes ill. There is no
controlling factor over the regeneration which occurs with evolution,
which is the origin of cancer. There is also a missing element,
causing something like anemia that is unrecognizable by modern
science. There is, however, a supplement which corrects this
situation. And no, it is not for sale. Contact me in person for more
information.

juggler - 
I appreciate that you listented to and summerized the audio file - I
found it very interesting and I hope you enjoyed it to.
-rpc