From these pairs of fossils, we can see the "eternal love" of creatures more than 40 million years ago

Messel Pit is a world heritage site in Germany. It was originally the bottom of a small volcanic lake, and it preserves a rich variety of complete animal and plant fossils, including many turtles with the scientific name of Allaeochelys crassesculpta. This turtle belongs to the family of Two-clawed Trionychidae. Interestingly, many turtles are in pairs, showing a "one on top of another" posture.

A. crassesculpta fossil specimen, SMF ME 2449 | Walter G. Joyce et al. / Biology Letters (2012)

By referring to its living relative, the pig-nosed turtle Carettochelys insculpta, we can determine the sex of the pair (males have long tails, females have short tails). It turns out they are doing exactly what you think they are doing, the only known example of a vertebrate fossil that maintains a mating position. A. crassesculpta died biologically about 47 million years ago, and now it is dying socially in front of humans.

Eternal Love

From fossils, we can observe all kinds of life details frozen in an instant, even "private affairs". A paper published in PLoS ONE in 2013 introduced an insect fossil discovered by Chinese scientists in Inner Mongolia: it was a pair of cicadas , facing each other ventrally, with the male's penis (aedeagus) still inserted in the female's copula (bursa copulatrix). The researchers named it Anthoscytina perpetua, and perpetua is Latin for "eternal love".

Forever frozen in time | Shu Li et al. / PLoS ONE (2013)

Sex at a "microscopic scale" can also be preserved in fossils. The oldest sperm fossils were found in a cave in Australia. These sperms came from ostracods dating back 23 to 16 million years. Ostracods are small animals of the subphylum Crustacea, commonly known as ostracods, and have the largest sperm in the animal kingdom. Some ostracod sperm are up to 1 cm long, four times their own length. However, the sperm of a fruit fly, Drosophila bifurca, is longer than that of ostracods, reaching 5.8 cm, because it has a very slender tail.

In the same cave, paleontologists also discovered fossils of female ostracods, which retained passages in their bodies to hold sperm . These passages are winding and longer than the sperm itself, and are truly "unfathomable."

A fossil of a female ostracod, Newnhamia mckenziana, preserves sperm storage organs and winding passages | Renate Matzke-Karasz et al. / Proceedings of the Royal Society B: Biological Sciences (2014)

Go to the underworld together

The moment of animal death preserved in fossils not only provides jokes, but also contains a lot of important information in paleontology and paleogeology.

The previously mentioned Messel Fossil Pit has a mystery: Who killed the animals here? There are two more reliable explanations: one is that geological activity at the bottom of the volcanic lake released toxic gases , and the animals were suffocated to death. The other is that toxic blue algae bloomed in the lake, and the animals who drank the water were poisoned and died.

Turtles that died during mating can provide us with some Conan-style clues. When modern aquatic turtles are courting, males generally make a series of movements in the water to convey courtship signals before starting to mount. The male and female will remain in the mounting position without moving. If mating occurs in open water, both turtles will sink into quite deep water.

Living pig-nosed turtles, whose smooth skin helps them absorb oxygen | Bjoertvedt / Wikimedia Commons

Cyanobacteria need sunlight to survive, so they are distributed in the surface layer of the lake. If there are cyanobacterial toxins, they should also be in the surface layer. Turtles cannot complete the courtship ritual in the toxic surface water. A.crassesculpta belongs to the Trionychia superfamily. The skin of this type of turtle lacks scales and has rich blood vessels in the epidermis, which can assist breathing, but also increases the possibility of absorbing toxins through the skin.

A more reasonable guess is that the volcanic gas at the bottom of the lake rose up but had not yet reached the surface water, so the lake water was in the form of a double cocktail: poisonous below and non-toxic above. The passionate A.crassesculpta completed the courtship ritual, but when mating began, they hugged each other motionless and sank into deeper water , where they were exposed to toxic substances and died together.

Who is the lover

The oldest "sexual fossil" was found in Nunavut, Canada. It is a fossil of red algae about 1 billion years ago , with spores of both sexes, indicating that it can reproduce sexually. Its scientific name is Bangiomorpha pubescens, and pubescnes means "adolescence", indicating that from it, the earth's creatures entered the 18-ban sexual era.

Who was the one who created love in the beginning of time? Biologists believe that the birth of sex not only adds fun to life, but also has far-reaching significance for the evolution of life .

Fossils of the red alga B. pubescens | Nicholas Butterfield / Paleobiology (2000)

One view is that sex can preserve the "design book" of life - genes . Asexual organisms such as bacteria can produce the next generation at any time, but sexually reproducing multicellular organisms select a portion of cells in the early stages of development for future sexual reproduction (for example, the "primary oocyte", the predecessor of human egg cells, is formed when the embryo is three months old). This portion of cells is protected to minimize division and external interference, thereby retaining complete genes and avoiding harmful mutations, just like a rare book preserved in a library.

Another point of view is more interesting. The role of sex is to make cells do a good job of "team building" . The survival of multicellular organisms requires the cooperation of many cells: for example, when there is nutrition, everyone shares it, and when encountering parasites, immune cells must be sent to resist foreign enemies. The problem is that under normal circumstances, natural selection does not favor united and selfless cells . If some "selfish" cells only care about reproducing themselves, the interests of the group will be damaged, and even the whole situation will be overturned (there is a kind of adverse phenomenon caused by "selfish" cells that we all know, called cancer).

The relationship between cells in multicellular organisms is very similar to that of an ant nest. Worker ants sacrifice their own reproduction to increase the number of reproduction of the queen ant | Pixabay

So, under what circumstances will natural selection benefit selfless cells? If the cell can choose to help its relatives , or its own copies, then even if it makes a sacrifice, because its relatives have the same "solidarity gene", the "character" of "solidarity" will still be preserved, and even more will be reproduced.

Multicellular organisms that reproduce sexually all develop from one cell (fertilized egg) , so except for a few mutations, all cells in the body have the same genes. In other words, this is an environment that helps natural selection produce "solidarity genes" because we are all one family.

B. pubescens is the earliest known complex multicellular organism with clear evidence, but it is not our ancestor. On the ancient branches of the biological evolution tree, "sex" and "complex multicellularity" have originated multiple times on different branches. Moreover, these two talents appeared very close in time, which seems to suggest that "sex" plays a key role in the emergence of multicellular life. After sex promoted the "team building" of cells, life was able to develop from a tiny single-cell form to a larger, smarter, more complex and colorful form, which has been thriving until today.

With sex, life enters a whole new level | Pixabay

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