How do we know dinosaurs existed if none of us on the Earth right now were alive when they existed? Fossils could have

Question: How do we know dinosaurs existed if none of us on the Earth right now were alive when they existed? Fossils could have been placed everywhere by humans but none of us would know it?
- Keywords:
  dinosaur,
  earth,
  fossil,
  human
Asked by Ayrton to Ed, Kerrianne, Nina, Oli, yoyehudi on 6 Nov 2017.
- Kerrianne Harrington answered on 6 Nov 2017:
  
  We have to use the evidence we are presented and a bit of occam’s razor, which means we make as few unnecessary assumptions as possible.
  
  It’s very unlikely that a human placed fossils everywhere because carbon dating can tell us how old something is. We use the radioactivity of carbon to find out the age of something that was once living, and we can do this with dinosaur bones. Whilst something is alive, it is exchanging carbon with the atmosphere. When it dies, it stops. The carbon material will radioactively decay over time, this means the amount of a particular carbon goes down over time. So we can date how long ago it was alive by looking at its radioactivity. So we’re pretty confident the fossils are as old as we think they are. So if they are older than humans, it’s difficult for a human to have hidden them to trick us.
- Oli Wilson answered on 6 Nov 2017:
  
  It’s quite unlikely that fossils could have been put in place by humans – the ones we mostly find have been hidden deep inside rocks, many of which formed under ancient seas. The other thing, which Kerrianne mentioned, is the dating. Carbon dating is useful, but going back even further you can use the radioactive patterns in rocks (rather than living tissue) to tell you how old they are – using combinations of uranium and lead (or even more exotic ones like samarium and neodymium) can let you accurately work out the age of something that’s billions of years old. You might find a fossil that’s surrounded by rocks 300 million years old – that’s 1000 times older than the oldest modern humans, so overall it’s basically impossible that humans can have put it there.
  Another way we know dinosaurs existed is… birds! All living birds are descended from dinosaurs, which you can see really clearly when you look at their skeletons (example here https://xkcd.com/1211/) – we know birds exist, and we can look at lots of bird ancestors all the way back to dinosaurs, so that can increase our confidence that dinosaurs actually existed too. If you want to have a look at the bird/dino family tree, click on the pictures in this tweet https://twitter.com/OliJWilson/status/900656031197474816

Question: How do we know dinosaurs existed if none of us on the Earth right now were alive when they existed? Fossils could have been placed everywhere by humans but none of us would know it?

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