Life on Earth first appeared when after being in its molten rock state for way too long, the Earth’s tectonic plates shift and released into the atmosphere and into the oceans elements favorable to life. But what exactly were the first living things to exist on our beloved planet? Scientists have been struggling with that question for quite a while, but now they have probably come up with an answer – the first animals on Earth were most likely sea sponges.
Algae vs. sea sponges
Finding a very large number of fossils dating back to roughly 540 million years ago, paleontologists realized that that they had all evolved from single celled organisms in a very short matter of time. This is known as the Cambrian explosion – an event that set in motion the eventual appearance of humans.
However, pre-Cambrian fossils are very strange, making it exceedingly difficult to ascertain the type of animal to which they belonged. Some of these fossils are in the form of molecules captured in ancient rocks, preserved almost intact after all this time.
So far, the theory had been that those molecules, which were dated to be far older than the Cambrian explosion, either belonged to algae or to sea sponges, as they were the most likely to predate other forms of multicellular life. But there really wasn’t any way to determine which one it was.
Uranium and cholesterol
That is, until the development of more modern ways of dating. Back in the ‘90s, a team of researchers including the lead author of the current study found traces of 24-isopropylcholestane, a form of cholesterol which is basically an ancient lipid. These traces were found in the aforementioned fossils.
Flash forward to 2009, when a team finally finished analyzing the rocks and determined they were around 640 thousand years old – the oldest evidence of animal life to date. This was determined by using modern dating techniques that use uranium-lead.
The “battle” raged on, as both algae and sponges are known to produce the 24-isopropylcholestane lipid, but the scientists still didn’t know which came first. The next part of the study consisted of identifying the gene responsible for generating the lipid, finding the organisms which carried it, and then trace when it evolved in them.
Finding the sterol methyltransferase gene, or SMT, responsible for the lipid, and present in both sponges and algae, the team went on with the dating techniques, and finally uncovered the truth – sea sponges developed the gene way before algae even appeared, some 640 million years ago.
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