Bones, teeth and even nails or hair are the main witnesses left from the past. Soft tissues, internal organs, muscles, tendons or skin do not cope well with the passage of time and fade. Faced with this reality, a group of archaeologists and paleontologists has compiled a sample with thousands of soft parts to prove that the brain is the best preserved of all. In a third of the cases it is not clear how they have been able to last up to 12,000 years. They believe that there is something in the gray matter that acts as a preservative.
After reviewing archives, archaeological databases and hundreds of published investigations, they gathered information on more than 13,000 soft parts from 213 sites around the world. Of them, 4,405 were human brains, which makes them the soft part that appears most in the sample. In fact, more than a thousand of them are the only soft remains of the bodies found. Such a quantity allows them to dismantle the idea that they are something rare in the fossil record. “Of course, a brain being preserved will be somewhat less common than if it is skeletal or dental tissues!” says Alexandra Morton-Hayward, forensic anthropologist at the University of Oxford (United Kingdom) and first author of this research published in Proceedings of the Royal Society B Biological Sciences. But he adds that “brains significantly outperform other tissues reported during the same time period.”
Placed on the map, there are fossil brains on every continent, except Antarctica. Most are concentrated in Europe, but they have also been preserved in other areas of archaeological tradition, such as the Andean mountain range, ancient Egypt, the Middle East or the extreme east of Asia. By adding the climate variable, they were able to investigate the causes of the conservation of tissues that, under normal conditions, should have disappeared within a few decades. “In the forensic field, it is well established that the brain is one of the first organs to decompose after death,” Morton-Hayward recalls.
The majority of brains (38%) have been preserved thanks to dehydration, generally by heat. For example, 500 remain from an Egyptian necropolis from 6,150 years ago. The other preservation mechanism has to do with soap, saponification. Under certain environmental conditions, the body's fats, adipose tissue, made up of 98% triglycerides, become adipocyre or cadaveric fence. This wax is what gives longevity to 30% of the brain tissues investigated. There are many, but they have only been found in seven sites, almost all of them (1,200) were found in a medieval cemetery in Paris. Two other avenues they have found are strikingly marginal. Only 1.6% of the brains they have located have been preserved by freezing in areas such as Alaska and high mountains, such as the Andes or the Alps. The tanning of the gray matter, as if it were lambskin, has only been found in about twenty cases, all in peat bogs in humid areas of Europe.
But there are 1,328 brains that have reached the present without knowing how, including the oldest in the sample, which are more than 12,000 years old. The authors of the study have classified them as having an “unknown” mechanism. It is not that they have not investigated it, it is that they do not fit into the four previous processes and in which they have not found specific environmental conditions that explain their conservation. They are also not helped by the fact that they were found in a wide variety of sites, including graves, barrows, shipwrecks, wooden and lead coffins, mass and shallow graves, fragmentary remains or severed heads. They do not share latitude or climatic conditions, although the presence of clay and iron in these places could explain some cases.
“There are few factors that brains of unknown type have in common, although waterlogging and the presence of clay and iron in the funerary environment are occasionally mentioned,” highlights Morton-Hayward. “What distinguishes them from other types of preservation is that they are the only soft tissue left, suggesting a mode of preservation unique to the central nervous system,” she adds. Indeed, of the 1,328 brain remains of unknown mechanism, 1,308 were the only soft remains found next to bones or teeth. “In addition, the absence of unifying environmental factors suggests that something in the brain itself (perhaps its biochemical composition in life) supports this unknown mechanism and facilitates its preservation after death,” concludes the British researcher.
That is going to be the second part of this story. They aim to find those endogenous elements or mechanisms that give greater longevity to brain tissue. It is true that the skull gives a temporary refuge to environmental action, but it also gives it to the heart or the tongue and, yet, no similar cases are preserved. Another element adds more mystery and reinforces the internal clue: on average, the sites where these brains have been found have higher levels of precipitation than other conservation mechanisms and humidity is one of the environmental factors that most accelerates the putrefaction of the material. organic. Among the possibilities they point out, their main hypothesis is that of a chemical interaction between various elements, such as between proteins and lipids, mediated by iron. All three are abundant in gray matter and would be the key to its conservation.
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