
Clotting of human blood plasma after addition of thromboplastin. A gel-like structure is formed that is strong enough to hold a steel ball. By Dietzel65, Steffen Dietzel (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
F3 is found throughout bony vertebrates, so it is at least 440 million years old.
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