We were lucky: the Earth It is the only planet in the solar system with the characteristics conducive to the birth of life, as we know it. Between them, her size.
If I was much older, like Jupiterthe atmospheric density would be so high that it would prevent the arrival of Sunlightmain source of energy for living beings. A smaller size, like Martenwould amount to not exerting enough gravitational attraction to retain an atmosphere and a hydrosphere that organisms would later enrich with oxygen.
The distance to the radiating star is also appropriate. Closer, as in Venusthe surface temperature would be so big than water, the medium in which metabolic reactionswould be in a state gaseous; further, as in Martenwould be very weak and would be freeze.
meteor bombardment
From 4.6 billion years (Ma), when the Earth was formed, until around 4000 Ma, its surface was subjected to heavy meteor bombardment. Among other collisions, the protoplanet Theiathe size of Martenwhich is the origin of Moon. This prevented the development of life during the hadic eon.
Although the fossil record does not indicate when the first organisms appeared, some geochemical evidence They suggest it happened in the Archaicabout 3,800 Ma ago. This is the age of the oldest known sedimentary rocks, the banded structures of iron and graphite alternating with flint of Isua training in Greenland. In this graphite, the heavy stable isotope of carbon (¹³C) is rareas in the organic material. The most conclusive evidence is stromatolitesbiogenic structures from autotrophic cyanobacteria it’s like that phototropismrecorded before 3500 Ma.
During Proterozoic discoveries continue, including the emergence of eukaryotic organisms. But it’s at the beginning of Cambrian period (542 Ma), already in the Fanerozoicwhen a good recording is available on the evolution of life: appear the mineralized skeletal tissuesincreasing the potential of fossilization of organisms.
The fossil record: a window to other worlds
The paleontology studies fossils from all angles to reconstruct life in the geological past. Therefore, it deals with historical eventsof nature contingent and irreplaceable. According to the geneticist Theodosius Dobzhansky“nothing makes sense in biology except in the light of evolution”.
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fossil hunting brings with it the uncertainty and excitement of resurrecting a creature never before seen by human eyes“
Moreover, the zoologist Ernest Mayer I affirm that without paleontology we could not solve certain evolutionary problems and many more that we can’t even imagine (think big disasters, like the who wiped out the dinosaurs after an Upper Cretaceous meteorite impact). The reason is that “they are only the paleontologistsamong all biologists, who have access to time dimension evolutionary phenomena.
These considerations led the paleontologist George Gaylord Simpson to assert that “fossil hunting involves the uncertainty and thrill of resurrecting a creature never before seen by human eyes, making us ponder the enigmas of the meaning and nature of life and man” .
The fossil record is full of examples of organisms we could never imagine existed, open a window to previous worlds.
Sea lilies: they look like plants but they are not
The bone crinoids They form a special group of animals. They belong to echinodermswhere they are flanked by sea urchins, starfish, brittlestars and sea cucumbers, as well as other classes already gone.
The anatomy of the stalked crinoids it is visible. These species, which are not very diversified and small sizeare known as sea lily. His body is made up of a Three segmented with are attached to the substrate (they are sessile or slightly motile), accompanied by a chalice (body) and articulated arms with cirrhosis which collect food (they are suspension feeders). Therefore, the first impression was that he was plants.
Although they can live at considerable depths, as animals they need presence of oxygen in the water. It is therefore unusual to find some fossil forms in oxygen-free medialike Jurassic crinoid Seirocrinus subangulariswhose peduncle reaches the 15 meters in lengthwith a calyx diameter and arms up to 80 cm.
How was such a thing possible? Cut in a kind of benthic and anoxic life, as evidenced by the sediments in which they are preserved? The key observation was made in 1968 by the German paleontologist Adolf Seilcherwho pointed out that the chalices They always appear located on the arms, while the stem is arranged on the calyxes. This suggested to him a more plausible hypothesis than the the larvae tied to driftwood logsdeveloping like pelagic organisms in the upper part of the water column, rich in nutrients, until its growth causes the sinking of logs. The arms would be the first to hit the bottom, conservation of articulated copies.
The proof that his explanation was correct came when find evidence of the lignitized trunks.
A: from bottom to top, sequence of colonization and development on a drifting trunk, specimens gradually reaching adult size. B: the weight of the crinoids causes the trunk to sink, settling in Paul Palmqvist Barrena
These crinoids were not the only ones to have a unusual lifestyle. Thus, some forms (Scyphocrinites) they could be floats thanks to a distal bulb (lobolite) in the bladder in its stem, or by rigid arms arranged radially (Saccocoma), while other small maybe swim actively (Uintacrinus).
A: Reconstruction of the cosmopolitan crinoid Scyphocrinites elegans, which lived between the Late Silurian and Early Devonian, whose column may have ended in a bulbous floating chamber (initially identified as a different genus, Carolicrinus). Image by BizleyArt. B: Fossil of Saccocoma tenella from the Jurassic. Image modified from a photo in the Cambridge University Press volume Fossil Crinoids. Paul Palmqvist Barrena
There are other worlds, but they existed in this
In short, although paleontologists are fossil huntersyou don’t kill prey to hang them as trophies on the wall. Our research brings fossils back to liferecreating with them the wonderful worlds that existed on Earth.
Today it is fashionable to invest huge resources in search of unlikely traces of life on other planetsas Marten. Perhaps we would be better off allocating some of these funds to learn more about our past. 
This article was written by Paul Palmqvist Barrenaprofessor of paleontology at University of Málaga. You can read the original article at The conversation.