Where is it claimed it appeared fully formed? 400,000 years is a long time with a lot of offspring. Assuming a generation of 30, a very high estimate, it’s like your grandparents with over 13,000 greats in front of grandparent.
Okay, point taken. I’ll be interested to see if they find any more fossils in that 400,000 year span and see any development of that feature in the hand.
Scatter (not bury) a dozen dead chickens in differing terrain around your vicinity with location coordinates noted and come back after a year (leave alone millions) to count how many have skeletal remains identifiably located. This alone will show you how rare an occurrence it is to have bones merely fossilise.