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The word âfossilâ is one that I type out rather frequently. Youâll often read stories about new hominin remains in which they are described as fossils. But hang on. Fossils take a long time to form, so how old does a human bone or tooth need to be before it counts as a fossil? Should we be hurling this word around with such gleeful abandon?
Let he who is without sin and all that: in my recent Paranthropus feature I cheerfully referred to bones from around 2 million years ago as fossils. That seems OK, but Iâve also talked about Asian fossils from a few hundred thousand years ago, and recently I said the Neanderthals âvanish from the fossil record about 40,000 years agoâ.
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Are these things really fossils, or are they just, well, old bones? The answer, it will not surprise you to learn, contains nuance.
Old fossils
To get my head around this, Iâve been exchanging emails with Jessica Thompson at Yale University. One of her interests is taphonomy: the study of how organisms decay â or are preserved or fossilised â over time.
Thompsonâs main message on when to define something as a fossil is, âit dependsâ. There are two primary questions: what conditions is a specimen deposited in, and what do we actually mean when we say âfossilâ? (By the way, just for simplicity, Iâm ignoring trace fossils like footprints preserved in dried mud. Weâre solely talking about actual bits of organisms.)
Letâs tackle the question of conditions first because itâs the less knotty of the two. A dead body, or part of a body, will decay or fossilise faster or slower depending on the conditions. The most obvious factor is temperature because cold slows down decay. Ătzi the mummified âicemanâ lived 5300 years ago but his body was found in remarkable condition because heâd been entombed in a glacier the whole time. If heâd been somewhere warmer, he would have rotted away. Temperature is also a big factor in the preservation of ancient DNA: the oldest DNA has been found in chilly places and we have little from tropical regions.
But there are plenty of other factors. Thompson lists some: âWhat kinds of sediments are surrounding them? What is the pH [acid or alkali]? What is the mineral composition? How much groundwater is there, and what kind of mineral content does it have?â
You can even get different levels of preservation within the same individual. âIf there is a burial in a cave and one bone has a drip over it that is constantly dripping mineral-rich water⊠it will fossilise differently from another part of the skeleton that is lying in a more dry area,â Thompson says.
The point here is that there isnât a consistent cut-off point for when something becomes a fossil because fossilisation proceeds at different speeds depending on the conditions. Now letâs consider what we actually mean by fossilisation.
Fossils and subfossils
The general idea is that the organic material in the body gets gradually replaced by minerals. So if you dig up a fossilised dinosaur bone, you arenât literally holding a piece of bone: youâre holding a rock that has taken on the form of the bone.
âA lot of scientists would consider a true âfossilâ to be a situation where there has been complete mineral replacement of all organic material in a bone, shell or other body part,â Thompson says.
This process is inherently gradual. âAt any point in time between original deposition and the time of discovery, a specimen will have become âmore fossilisedâ than it previously was, until complete mineral replacement is achieved,â she says.
So then the question becomes: where do you want to draw the line? Nature doesnât give us a clear answer for this.
Thompson talks about âsubfossilsâ, which have lost most of their organic matter but havenât undergone full mineral replacement. She also describes fossils that are variously âheavyâ or âlightâ depending on how much mineral has become incorporated. These distinctions can be quantified, but exactly where you decide to call something a fossil or âfully fossilisedâ or whatever is a judgement call.
Fresh bones
Letâs finish by making this more concrete. What does this mean for actual hominin remains?
Weâre dealing with a period of 7 million years: thatâs the age of the oldest known hominin, Sahelanthropus. Australopithecus, which includes the Lucy specimen, lived around 3 million years ago. The oldest members of our genus, Homo, are maybe 2.5 million years old. Neanderthals, Denisovans and our species, Homo sapiens, are all from within the last million years. The Neolithic period, which saw the rise of farming and sedentary living, is all within the last 10,000 years.
This is exactly the period in which the different gradations of fossilisation appear. In other words, some hominin remains are more fossilised than others.
Thompson offers a very rough guide. Say you define a fossil as âcomplete mineral replacement with no remaining organics at allâ. Because this happens at different rates, it can take anywhere from 10,000 to 6.5 million years, she says. Thatâs the entire hominin timespan. In other words, itâs theoretically possible to find remains of the very earliest hominins that, by this metric, arenât fully fossilised.
On the other hand, you might want to define it as a mild degree of fossilisation, say âsubstantial mineral replacement and loss of most organicsâ. In that case, Thompson says most bones and teeth would be fossilised if theyâre older than about 30,000 years. Furthermore, plenty of Neolithic bones that are only a few thousand years old have gone at least some way down this path.
This has a curious and poignant consequence when you consider recent evolutionary history. Homo sapiens has been the sole hominin species since about 40,000 years ago, when the Neanderthals died out. Other groups like the Denisovans, âhobbitsâ (aka Homo floresiensis) and Homo erectus seem to have gone even earlier.
This means the hominin extinctions all happened just a little too long ago for us to have much hope of finding unfossilised remains. It might be possible to find some Neanderthal bones that have fossilised unusually slowly and are still somewhat âfreshâ, but weâd have to be lucky: while 40,000 years isnât a huge span of time compared with the full hominin timeline, itâs long enough for fossilisation.
In other words, ours is the only species for which we are likely to find unfossilised remains. The others have turned to stone.
