1 00:00:02,080 --> 00:00:07,400 Speaker 1: Welcome to brain Stuff from how stuff works. Hey, brain stuff, 2 00:00:07,440 --> 00:00:10,799 Speaker 1: it's me Christian Seger. You know, when I'm digging a 3 00:00:10,840 --> 00:00:13,160 Speaker 1: six foot hole in the middle of the desert, I 4 00:00:13,200 --> 00:00:16,600 Speaker 1: start to wonder where are all the dead animals? Shouldn't 5 00:00:16,600 --> 00:00:19,360 Speaker 1: we be waiting knee deep in fossils every time we 6 00:00:19,440 --> 00:00:22,520 Speaker 1: go outside. I know that's morbid, but you can probably 7 00:00:22,560 --> 00:00:27,600 Speaker 1: guess that not every animal that dies leaves behind fossil evidence. 8 00:00:27,640 --> 00:00:30,880 Speaker 1: But why is that? Well, just to get our terms straight, 9 00:00:30,920 --> 00:00:34,760 Speaker 1: a fossil is any physical remnant left behind by an 10 00:00:34,880 --> 00:00:38,959 Speaker 1: organism that died long ago. In many cases, fossils might 11 00:00:39,040 --> 00:00:43,120 Speaker 1: only be things like preserved footprints or nest sites. But 12 00:00:43,200 --> 00:00:47,400 Speaker 1: today we're looking at direct remains of animal bodies, like bones. 13 00:00:47,800 --> 00:00:51,960 Speaker 1: The likelihood that any particular animal body will become fossilized 14 00:00:52,120 --> 00:00:55,959 Speaker 1: is amazingly small. It's actually less than one percent. So 15 00:00:56,120 --> 00:01:00,640 Speaker 1: let's look at the stations of the obstacle course to fossilization. First, 16 00:01:00,720 --> 00:01:04,720 Speaker 1: there's body type. Fossilization has a strong preference for animals 17 00:01:04,720 --> 00:01:09,479 Speaker 1: with hard body parts like bones, teeth, and shells. Animals 18 00:01:09,480 --> 00:01:12,959 Speaker 1: with soft bodies like slugs and jellyfish, well, they usually 19 00:01:13,000 --> 00:01:16,760 Speaker 1: just decompose completely and disappear after death, and except in 20 00:01:16,840 --> 00:01:21,520 Speaker 1: a very few rare cases like freezing, dry mommification and 21 00:01:21,640 --> 00:01:25,119 Speaker 1: peat bog preservation, the same thing happens to the soft 22 00:01:25,160 --> 00:01:30,360 Speaker 1: tissues on all animal bodies, skin, organs, eyeballs, et cetera. 23 00:01:30,680 --> 00:01:34,559 Speaker 1: They all make excellent meals from microorganisms and are thus 24 00:01:34,600 --> 00:01:38,480 Speaker 1: consigned to the ravages of rot. The second main hurdle 25 00:01:38,520 --> 00:01:42,640 Speaker 1: defossilization is exposure. To become a fossil, you need to 26 00:01:42,680 --> 00:01:45,640 Speaker 1: be one of the rare animal bodies that is rapidly 27 00:01:45,680 --> 00:01:49,840 Speaker 1: buried soon after the animal dies. This is most likely 28 00:01:49,920 --> 00:01:52,559 Speaker 1: to happen in or near the site of a moving 29 00:01:52,600 --> 00:01:55,680 Speaker 1: body of water, like a river or a floodplain, where 30 00:01:55,760 --> 00:01:59,800 Speaker 1: runoff floodwaters or regular flow may quickly cover a dead 31 00:02:00,040 --> 00:02:04,280 Speaker 1: body in sediment. It might also happen in arid desert 32 00:02:04,360 --> 00:02:08,440 Speaker 1: settings where wind can quickly bury animal remains in sand dunes. 33 00:02:08,880 --> 00:02:13,120 Speaker 1: If the remains are not rapidly buried, scavenging animals are 34 00:02:13,200 --> 00:02:17,040 Speaker 1: likely to scatter and then consume them. After all, nature 35 00:02:17,160 --> 00:02:19,799 Speaker 1: hates to pass up a free lunch, and even a 36 00:02:19,840 --> 00:02:23,959 Speaker 1: clean skeleton left out exposed to the elements will eventually 37 00:02:24,000 --> 00:02:29,320 Speaker 1: be erased by the ravages of the weather. That's decalcification, erosion, 38 00:02:29,680 --> 00:02:33,640 Speaker 1: and corrosion. But let's say your bones are lucky enough 39 00:02:33,639 --> 00:02:37,080 Speaker 1: to be rapidly buried somehow. The next big hurdle is 40 00:02:37,080 --> 00:02:41,120 Speaker 1: the sediment itself. A nice dry sand or alkaline mud 41 00:02:41,200 --> 00:02:43,480 Speaker 1: might be a good place to become a fossil, but 42 00:02:43,520 --> 00:02:46,120 Speaker 1: if your bones are buried in soil with a higher 43 00:02:46,120 --> 00:02:50,239 Speaker 1: temperature and higher acidity, your prospects are a lot slimmer. 44 00:02:50,560 --> 00:02:53,920 Speaker 1: Acidic environments meaning soils with a low pH tend to 45 00:02:53,960 --> 00:02:58,280 Speaker 1: dissolve hydro zappatite, a calcium phosphate mineral that is a 46 00:02:58,360 --> 00:03:02,480 Speaker 1: main structural ingredient in our bones. So many soil types 47 00:03:02,520 --> 00:03:06,679 Speaker 1: on Earth will simply destroy all the bones they swallow. 48 00:03:07,000 --> 00:03:10,960 Speaker 1: But even in friendly sediment, over a long enough period 49 00:03:11,000 --> 00:03:14,520 Speaker 1: of time, bones can break down. The organic proteins and 50 00:03:14,600 --> 00:03:19,480 Speaker 1: bones like collagen, eventually decompose, and the inorganic molecules and 51 00:03:19,560 --> 00:03:24,440 Speaker 1: bones can be crushed, dissolved, or otherwise destroyed by physical 52 00:03:24,520 --> 00:03:28,519 Speaker 1: force over the centuries. So if you want your actual 53 00:03:28,639 --> 00:03:31,800 Speaker 1: bone structure to survive, you have to be lucky enough 54 00:03:31,840 --> 00:03:36,680 Speaker 1: to undergo a little transformation. Most really ancient bones we find, 55 00:03:36,720 --> 00:03:40,320 Speaker 1: such as dinosaur bones, aren't the unaltered original bones that 56 00:03:40,360 --> 00:03:44,320 Speaker 1: were buried millions of years ago. Instead, they're either a 57 00:03:44,440 --> 00:03:49,880 Speaker 1: minerally modified versions of those bones or be stone photocopies. 58 00:03:50,320 --> 00:03:54,920 Speaker 1: Two processes represent the majority of these cases, perma mineralization 59 00:03:55,080 --> 00:04:00,480 Speaker 1: and replacement. In perma mineralization, mineral rich waters seep into 60 00:04:00,480 --> 00:04:02,960 Speaker 1: the buried bones and fills the pores of the bones 61 00:04:03,160 --> 00:04:07,520 Speaker 1: with its mineral content. These minerals form crystals inside the bones, 62 00:04:07,600 --> 00:04:11,120 Speaker 1: causing them to modify and harden over time. Sometimes this 63 00:04:11,200 --> 00:04:15,760 Speaker 1: process is also called petrification. In replacement, the original bones 64 00:04:15,760 --> 00:04:20,000 Speaker 1: can be completely dissolved but still leave fossil copies, as 65 00:04:20,040 --> 00:04:24,480 Speaker 1: the mineral in the groundwater completely replaces the shape of 66 00:04:24,480 --> 00:04:28,640 Speaker 1: the bones over long periods of time. So let's say 67 00:04:28,680 --> 00:04:32,520 Speaker 1: you're the rare dead animal that wins the fossilization lottery 68 00:04:32,520 --> 00:04:35,919 Speaker 1: and you just happen to pass all these tests, you 69 00:04:35,960 --> 00:04:38,640 Speaker 1: still have to be found. The total surface of the 70 00:04:38,680 --> 00:04:42,520 Speaker 1: Earth is almost two hundred million square miles, and even 71 00:04:42,640 --> 00:04:45,200 Speaker 1: for a guy like me, there's only so much time 72 00:04:45,240 --> 00:04:53,360 Speaker 1: to dig. Check out the brain stuff channel on YouTube, 73 00:04:53,440 --> 00:04:55,599 Speaker 1: and for more on this and thousands of other topics, 74 00:04:55,760 --> 00:05:10,720 Speaker 1: visit how stuff works dot com.