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Asteroids and dinosaurs
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  Three observations, one hypothesis
The KT boundary layer contained plenty of iridium but no plutonium-244. Furthermore, the boundary marked what seemed to be a major extinction event for marine and terrestrial life, including the dinosaurs. What hypothesis would fit all those disparate observations and tie them together so that they made sense? The team came up with the idea of an asteroid impact — which would explain the iridium (since asteroids contain much more iridium than the Earth's crust) and the lack of plutonium — but which also led them to a new question: how could an asteroid impact have caused the dinosaur extinction?

At this point the asteroid hypothesis is supported by two lines of evidence, but it's unclear how an asteroid would have caused extinctions

Once again, Luis Alvarez came to the rescue with some calculations and an elaborated hypothesis. Talks with his colleagues led him to focus on the dust that would have been thrown into the atmosphere by a huge asteroid impact. He hypothesized that a huge asteroid had struck Earth at the end of the Cretaceous and had blown millions of tons of dust into the atmosphere. According to his calculations, this amount of dust would have blotted out the sun around the world, stopping photosynthesis and plant growth and hence, causing the global collapse of food webs. This elaborated version of the hypothesis did indeed seem to fit with all three of the lines of evidence available so far: lack of plutonium, high iridium levels, and a major extinction event.

If the hypothesis is that an asteroid struck and caused a dust cloud then we'd expect to see a major extinction at the end of the Cretaceous in the fossil record.  That is actually what we see.

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