Welcome to brain Stuff from how stuff works. Hey, brain stuff, it's me Christian Seger. You know, when I'm digging a six foot hole in the middle of the desert, I start to wonder where are all the dead animals? Shouldn't we be waiting knee deep in fossils every time we go outside. I know that's morbid, but you can probably guess that not every animal that dies leaves behind fossil evidence. But why is that? Well, just to get our terms straight, a fossil is any physical remnant left behind by an
organism that died long ago. In many cases, fossils might only be things like preserved footprints or nest sites. But today we're looking at direct remains of animal bodies, like bones. The likelihood that any particular animal body will become fossilized is amazingly small. It's actually less than one percent. So let's look at the stations of the obstacle course to fossilization. First, there's body type. Fossilization has a strong preference for animals
with hard body parts like bones, teeth, and shells. Animals with soft bodies like slugs and jellyfish, well, they usually just decompose completely and disappear after death, and except in a very few rare cases like freezing, dry mommification and peat bog preservation, the same thing happens to the soft tissues on all animal bodies, skin, organs, eyeballs, et cetera. They all make excellent meals from microorganisms and are thus consigned to the ravages of rot. The second main hurdle
defossilization is exposure. To become a fossil, you need to be one of the rare animal bodies that is rapidly buried soon after the animal dies. This is most likely to happen in or near the site of a moving body of water, like a river or a floodplain, where runoff floodwaters or regular flow may quickly cover a dead body in sediment. It might also happen in arid desert settings where wind can quickly bury animal remains in sand dunes. If the remains are not rapidly buried, scavenging animals are
likely to scatter and then consume them. After all, nature hates to pass up a free lunch, and even a clean skeleton left out exposed to the elements will eventually be erased by the ravages of the weather. That's decalcification, erosion, and corrosion. But let's say your bones are lucky enough to be rapidly buried somehow. The next big hurdle is
the sediment itself. A nice dry sand or alkaline mud might be a good place to become a fossil, but if your bones are buried in soil with a higher temperature and higher acidity, your prospects are a lot slimmer. Acidic environments meaning soils with a low pH tend to dissolve hydro zappatite, a calcium phosphate mineral that is a main structural ingredient in our bones. So many soil types on Earth will simply destroy all the bones they swallow.
But even in friendly sediment, over a long enough period of time, bones can break down. The organic proteins and bones like collagen, eventually decompose, and the inorganic molecules and bones can be crushed, dissolved, or otherwise destroyed by physical force over the centuries. So if you want your actual bone structure to survive, you have to be lucky enough to undergo a little transformation. Most really ancient bones we find, such as dinosaur bones, aren't the unaltered original bones that
were buried millions of years ago. Instead, they're either a minerally modified versions of those bones or be stone photocopies. Two processes represent the majority of these cases, perma mineralization and replacement. In perma mineralization, mineral rich waters seep into the buried bones and fills the pores of the bones with its mineral content. These minerals form crystals inside the bones, causing them to modify and harden over time. Sometimes this
process is also called petrification. In replacement, the original bones can be completely dissolved but still leave fossil copies, as the mineral in the groundwater completely replaces the shape of the bones over long periods of time. So let's say you're the rare dead animal that wins the fossilization lottery and you just happen to pass all these tests, you
still have to be found. The total surface of the Earth is almost two hundred million square miles, and even for a guy like me, there's only so much time to dig. Check out the brain stuff channel on YouTube, and for more on this and thousands of other topics, visit how stuff works dot com.