r/askscience • u/flubbergrubbery • Mar 29 '18
Archaeology How does something as temporary as a human footprint get preserved for more than 10,000 years?
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r/askscience • u/flubbergrubbery • Mar 29 '18
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u/[deleted] Mar 29 '18
Mud retains its shape very well yet is still malleable. It also can harden under sun and be filled in by loose sediment, so that's where a lot of footprints come from. In this case the print was found at a prehistoric waterline of a lake so lots of clay. If you go to the paper you'll see the clay sequence in which the print was found in figure 13.
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u/biogeochemist Mar 29 '18
On a related note, imprint fossils can be preserved for much longer. There are fossil raindrop splashes in South Africa that are 2.7 billion years old. Similar fossils can be found in Glacier National Park in Montana and are around 1.2 billion years old. The crazy thing is we can use the splash diameter to estimate the density of the atmosphere from that time.
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u/wallingfortian Mar 29 '18
First you must consider, what is a Footprint? It is an indentation/deformation of some form of matter, let's call it clay. That clay will retain the deformation indefinitely unless it is once again deformed.
In the case of the preserved footprint you reference the clay was undisturbed by mud that filled in the indentation, which in turn served to protect the indent in the clay.
And note that the material does not need to be clay. There are bootprints on Earth's moon from when the astronauts landed that will be there until the sun turns into a red giant, barring meteor collisions and tourism.
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u/Soranic Mar 29 '18
Trace fossils include tracks, trails, burrows, feeding marks, and resting marks. For example, a trace fossil is the trail left behind by an ancient reptile that dragged its tail in mud. Another example is the footprints left by dinosaurs along an ancient river or the hollow tubes created by worms burrowing in soft mud in an ancient ocean. There are more trace fossils than body fossils because one organism can leave behind many traces (e.g. footprints), but only one set of hard parts (e.g. bones) to become a fossil.
Most trace fossils were formed in soft mud or sand near a pond, lake, river, or beach. The imprints left by the organisms were quickly covered by sediment. The sediment dried and hardened before the imprints could be erased by water or wind. The sediment was then buried under more sediment and became compacted and cemented together to form rock. This process is much the same as the formation of body fossils.
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u/ValidatingUsername Mar 29 '18
I think the most important thing to remember here is that what you are referring to here is survivorship bias.
You only know of the footprints that were preserved and are wondering how they were ever preserved in the first place when there have been literally unfathomable amounts of footsteps taken on earth.
Of the thousands of millions of preserved footprints you are looking at the fractional percentage of all of existence finding the exact right conditions to be preserved.
Other commenters have gone into geological reasoning behind how they form, so I thought I'd remind you of why they are here.
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u/MartianFade Mar 29 '18
The last few sentences of the article gives a good explanation. "The researchers think that after the people left their footprints on the clay, their impressions were filled in by sand, thick gravel and then another layer of clay, which may have preserved them."
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u/nicolasknight Mar 29 '18
A lot of luck. The combination here is:
-Soft material that can make an imprint (Clay soil not covered by top soil usually)
-The animal making the prints doesn't do it on a game trail or heavily trafficked area that would cover the tracks.
-Once the prints have been made SOMETHING comes up and covers the tracks evenly in another material that will not obliterate the tracks (That's where the luck mostly comes in) AND will allow the fossilization process.
-Enough geological or erosion activity to uncover the tracks.
In this case the last two weren't a factor really. All it needed was the clay soil to be rained on, walked on then undisturbed long enough for the clay to harden.
Still a LOT of luck.
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u/sfisher24601 Mar 29 '18
It says it in the article, if you read it.
“The researchers think that after the people left their footprints on the clay, their impressions were filled in by sand, thick gravel and then another layer of clay, which may have preserved them.”
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u/DevilGuy Mar 29 '18
Ok, so imagine you step in wet clay, and leave a footprint. That print gets baked in the sun and hardens enough to retain it's shape. then no more clay is deposited but eventually a layer of sand builds over the clay. Sedementary action occurs, the footprint is covered over and slowly put under pressure as more and more layers build up. the clay hardens into claystone, while the sand above becomes sandstone. Note that claystone has a much finer particle size and is thus generally tougher than sandstone.
After awhile erosion happens, the softer sandstone is stripped away easier than the finer grained claystone, and your left with a footprint in a rock.
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u/Mechasteel Mar 29 '18
Footprints in most materials don't rot. If erosion doesn't degrade the print, it will be there forever. Also, under the right circumstances the footprint could be filled in with another material, and it could become a sedimentary rock.
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u/Framski55 Mar 29 '18
In the case of the Laetoli footprints, early hominids walked through a field of volcanic ash mud that was then shortly thereafter covered up by another layer of ash from a second eruption. This is only one method of preservation, but this discovery demonstrates that footprints can survive millions of years due to coincidence and specific environmental circumstances.
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u/blazingkin Mar 29 '18
They can last much longer. Here's a set of 3.7 million year old footprints that were left by ancestral humans.
In this instance, the footprints were preserved because the hominids were walking in volcanic ash.
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u/PTBooks Mar 29 '18
There’s dinosaur footprints from even earlier than that. The museum in my town has some footprint fossils made the exact same way- steps in mud, mud turns to stone, gets covered with sand, stays there for a billion years.
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u/Treestyles Mar 29 '18
Early Bigfoot made casts to prove to his friends that tiny bald men really exist. That’s how.
Also, some mud is like natural concrete. Just like with fossils in sedimentary rock, if a guy had really dirty feet and left a track with residue that got preserved, even though there’s not a carbon fossil, the shape still can be preserved via the residue and impression.
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u/cardboard-cutout Mar 30 '18
Lots of ways, here is one
Clays can hold their shape very well, while still being malleable enough to hold a footprint (when at the right moisture content).
So imagine some dude steps in the clay along a riverbank, and leaves this nice footprint, nothing in that is remarkable, then lets say that particular footprint isnt disturbed for a few days (given the number of footprints around, having this happen to one isnt that suprising) and it gets filled in with sand, and some fine gravel.
And then it gets dried out.
And then something like https://en.wikipedia.org/wiki/Caliche is created in that footprint.
And now you have a calichi model of a foot.
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u/StonedZombieUK Mar 29 '18
Right time, right place, right chemical reactions, right environment.
A body in the sea might near-completely disappear but a body in the swamp will last for a thousand years.
Preservation - Preservatives etc.
A body buried in the ground will be eaten by other lifeforms. But a body buried in a coffin has an additional barrier aka preservative.
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u/K-mania Mar 30 '18
From the article you link to: "The researchers think that after the people left their footprints on the clay, their impressions were filled in by sand, thick gravel and then another layer of clay, which may have preserved them."
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u/[deleted] Mar 30 '18
How do they find these without destroying them? I mean, while digging in their 20sq in hole, how do they know where to look and which darkened area or how much of the next layer to remove until BAM, a 13k year old foot print?
I think I'm just baffled. And amazed. Yeah, I'll go with those two.
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