Getting a feel for last universal common ancestor
All living things originated from a common ancestor called the last universal common ancestor (LUCA). Let’s put this in perspective from our human evolution history:
Suppose you took your DNA and compared it to a Neanderthal. You’d find that both your DNA is 99.7% identical.
Next, if you compare your DNA to a chimpanzee, you’d see more differences. In fact, there’s about a 98.5% overlap. For a chicken, it’s 60%…
As you keep comparing, it never reaches a point where you don’t share any DNA. Simply, we share commonalities for our genetic composition and DNA with all organisms on Earth.
So back to LUCA which is the common ancestor that unites us all. By comparing the genomes of simple bacteria with eukaryotes, LUCA has the most universal genes in all life.
LUCA is not the origin of life
It’s a common misconception to think that LUCA is the origin of life. But the origin of life existed long before LUCA.
It’s believed the origin of life happened 3.5 to 4.1 billion years ago. Whereas the last universal common ancestor is thought to have lived 3.5 to 2.5 billion years ago.
During the 500 million years that separates LUCA and the origin of life, DNA had to evolve into a somewhat functional system.
For example, DNA included replication enzymes, transfer RNA and ribosomes at this time. This is because all living things share these same functions meaning they must have been present in LUCA.
LUCA thrived in underwater hydrothermal vents
If you look back at Earth’s early atmosphere, it didn’t contain oxygen. So this means LUCA likely was an obligate anaerobic organism.
LUCA also didn’t rely on other organisms. This is simply because there weren’t a lot around at this point in geologic time.
So it likely thrived in an iron-sulfur rich lifestyle in underwater hydrothermal vents
This was the type of environment that could provide water rich in hydrogen, carbon dioxide and other minerals for it to develop.