Prathyu: March 2011

For over 100 million years, dinosaurs, and not mammals, were the dominant form of life on Earth. The pinnacle of evolution at the time, dinosaurs filled the niches of being the largest, most differentiated animals -- herbivores and carnivores both -- on the planet. As you well know from seeing their fossils, they would dwarf their modern, mammalian counterparts if they were still alive today.

But they're not still alive today, because a mass extinction event occurred 65 million years ago. And the fossil record indicates that it occurred all at once, which is unusual. The Earth was overrun with dinosaurs, everywhere. In just a few hundred thousand years, they were all gone. All of them. Large, small, it doesn't matter. That's what extinct means -- every last one.

But one of the best science questions ever -- and this is always true -- is how? How did the dinosaurs, and many, many other forms of life, all become extinct at once? When we look at rocks, we find that one type -- sedimentary rock, forms from deposits. These deposits make it relatively easy to date the rock, or to figure out when the rocks (and hence the fossilized organisms inside) were deposited there. Here's what sedimentary rock looks like.

Well, there's a layer of rock, known as the K-T boundary, that seems to have a plethora of dinosaur fossils below the boundary, but almost none above the boundary. What's very interesting is that we find this exact same boundary all over the world, dating to 65 million years ago, with dinosaurs found abundantly below it, with the few ones found above it thought to be geologically reworked (upturned and reburied).

This thin layer of ash -- the boundary between light and dark in the above picture -- is the split between when dinosaurs are found and when they aren't. What's amazing about this is that this tiny, thin film of a layer contains something in great abundance that the Earth doesn't have much of at all: the element iridium. Naturally, the Earth has so little iridium that it's inconceivable that this iridium could have come from Earth. Where could it have come from?

Space. If a large enough asteroid or comet hit the Earth, the collision would spew up whatever elements were on that foreign projectile into the atmosphere, and spread it all over the globe. It could do the following things:

Block a lot of sunlight,
Disrupt the food chain from the bottom,
Cause mass extinctions all at once,
Cause tsunamis and environmental disasters, and
Leave a thin-layer deposit of minerals not necessarily found on Earth.

Well, guess what we observe? A huge extinction in photosynthetic organisms on land everywhere and even in the oceans. What else? The practically immediate extinction of nearly all non-avian dinosaurs and all animals over a certain size. What else? Geological evidence of tsunamis, flooding, and ashy deposits. And what else? That layer of iridium-rich ash found all over the Earth. Seems like all we need is a crater the right size that's 65 million years old, and that will be our "smoking gun" for this theory.