The genetics of extraterrestrial life

Recently someone shared a link about scientists at Sheffield University discovering extraterrestrial life. The scientist captured diatom frustules at 25 km above ground and they argued that it was impossible for it to have come from Earth and that it made more sense that it comes from the watery environment of a comet. In a different report it was suggested that certain structures in meteorites resembled cyanobacterial microfossils and that it was impossible that this was contamination from planet Earth.

 

I believe it is pretty much certain that life originated more than once in this Universe and I would dare to say that we will find life in other places from the Solar System, such as Mars, Europa, Enceladus and even Titan. However, if we discover life in other planets there are a few predictions or assumptions that we can make about their genetics that could give us a better understanding of the origin of life on Earth and the Universe. These predictions are based in what we know about the evolution and diversification of life on Earth.

Case one – Life originated somewhere else in the universe and seeded planet Earth with life.

One of the things that we know about life on Earth is that every single organism in this planet so far discovered, be it a bacterium, an ant, a fungi, or people, is descendant from a common ancestor, which originated (or came to Earth) around 3.8-4.0 billion years ago.

We know this because we share the same genetic system, same molecules of DNA, RNA, proteins… we share many genes and proteins that fulfill similar roles, the back bone of our metabolism is pretty much the same for all life on Earth. This means that IF life arrived to Earth from somewhere else, it did so only once 3.8 to 4.0 billion years ago and only one linage, a unicellular organism, gave rise to all the diversity we know now.

Every single organism we know, cyanobacteria and diatoms, all plants, all sorts of bacteria, all mammals and insects, all fishes and birds, all originated and evolved in planet Earth and nowhere else.

Let’s take the case of diatoms, diatoms are phytoplankton, they are marine algae. We know from fossils and genetics that diatoms originated around the Jurassic time, that is approximately 185 million years ago… possibly they have an earlier origin no further than 250 million years ago. This means that they evolved only recently in geological time. Diatoms are eukaryotes; since they are algae they do photosynthesis. The chloroplast of diatoms originated from a process called secondary endosymbiosis where the ancestral eukaryote diatom stroke a partnership with a red algae… red algae are also eukaryotes that do photosynthesis and also had a chloroplast, which originated in a process called primary endosymbiosis in which the ancestral red algae stroke a relationship with a cyanobacteria. Red algae and plants have one common ancestor. Thus we can trace the full history of diatoms based on the genetics of its genome, its chloroplasts, and the fossil record to events that are only possible to have occurred on planet Earth, and in consequence we can be pretty sure that they originated on planet Earth and nowhere else.

Diatom frustules

If the diatoms in the study at Sheffield University came from outer space they might have been kicked out from planet Earth by some mechanism in the last 250 million years and then returned. Maybe the meteorite that killed the dinosaurs sent some debris to space and stayed orbiting around the planet or the inner solar system. The same thing can be said of cyanobacteria fossils in meteorites, cyanobacteria are earthlings we are absolutely sure due to genetic studies and the fossil record that they originated and diversified on Earth.

Case two – Life as we know originated on Earth but we discover life somewhere else that had a different origin.

Let’s say that we send a mission to Jupiter’s moon Europa and in the inner oceans we find life. Let’s assume that life in this moon originated separately from life on Earth, thus it is a second origin of life independently from Earth’s origin. Or say for example we discover a comet that originated from another part of the galaxy where life evolved as well, independently from life in this planet, a completely different origin. What can we expect then from this life form?

IF life in these samples has a different origin it must then have a completely different genetic system to the one we find on Earth. The first expectation should be that it might not have DNA or RNA as genetic information molecules, but other completely different type of nucleotides or chemical compounds. Say that by coincidence the ingredients for life in this foreign moon or planet were similar to the ingredients of life on Earth so they also have DNA or RNA as their molecules to carry information. Then we should expect that the genetic code and the amino acids that they encode are of a completely different sort to the ones on Earth. There are hundreds of different amino acids; however Life on Earth use 20 very specific ones encoded in a very conserved genetic code. The chances that life which originated independently in another region of the universe uses the same genetic code to encode the very same 20 amino acids that life uses on Earth is rather small. 

But then less assume that just by chance, this independent origin of life used the same nucleotides and amino acids as it did on Earth… since it originated and evolved somewhere else at the very least every single gene, given the chance that this life also has “genes”… their sequences have to be radically different to the ones on Earth. The metabolism and physiology of these extraterrestrial organisms must be of a completely different type, optimized and specialized for the environments found in the moon or planet where it evolved. Just minute differences in the light intensity of the sun, the rotation of that planet (or moon), the weather, the different concentration of gases in the atmosphere or the salts of its oceans will lead to completely different sorts of genetics and biology to that on Earth, and more necessarily so if it originated separately and independently from Earth.

Thus even if the probability that life originated independently in a different planet is high, the chances that it evolved to exactly the same organisms as in planet Earth, the chances that a cyanobacteria or a diatom sprung from that separate origin, sharing thousands of the same genes, encoding for exactly the same proteins that exist on planet Earth are pretty much none. Unless they come from a parallel Earth in a parallel Universe.

Case three – Life on Earth originated somewhere else and seeded not only Earth but many other planets and places in the universe, and then now we find a comet with a life from another planet but sharing the same origin of life with us.

As I mentioned before ALL life on Earth that we know of descends from a common ancestor 4.0-3.8 billion years ago. This does not change, this is a fact.

Let’s then assume that life originated before, for example 6.0 billion years ago and somehow it spread throughout the universe… one linage from this origin seeded Earth 4.0-3.8 billion years ago. This implies that life on Earth have been evolving independently for billions of years.

Say now that the extraterrestrial life we found, before arriving to Earth, landed billions of years ago in another planet or moon, it thrived there evolving into many different forms and diversifying, then something happened and a sample of life from that planet reached Earth today. Thus, we should expect that since this extraterrestrial organism shared a common ancestor with terrestrial life at least 4.0 billion years ago it shares the same genetic system that life on Earth: DNA, RNA, proteins composed of the same amino acids, even similar cellular structures. HOWEVER, because Earthlings and extraterrestrials evolutionary paths separated before life diversified on Earth, this implies that every form of life on earth is more similar to each other, or shares a more recent common ancestor, that this Earthling ancestor is to the extraterrestrial organism. When comparing their DNA or proteins in an evolutionary tree, we must see that all life on Earth should make one branch and the extraterrestrial a separate one as in the tree I show below. This should be evident from their genetic sequence. Since the earthling and the extraterrestrial share a common ancestor they might share many traits, yet because the extraterrestrial organism has been thriving in another planet it should have also many unique characteristics that evolved to adapt itself to the environment it lived.

Phylogenetic tree of a hypothetical extraterrestrial organism sharing a common ancestor with all Earthlings

There is another possibility: the sample of life we find in the meteorite or in space might be the same as the common ancestor of all earthlings, but it had been frozen in space for billions of years and now we find it. Because all life on Earth shares many characteristics, we know what traits the common ancestor must have had. Thus we should find these characteristics in the extraterrestrial sample. This should also be evident from the sequence information of its genome.

In conclusion, if we find life out there we will know for sure once we examine their genetics and their characteristics. Also if we find cyanobacteria or diatoms in space they must have originated in planet Earth one way or another, no doubts about it.