From LUCA to Humans

30.06.2021 |

Episode #3 of the course The evolution of life in the universe by Silvano P. Colombano Ph.D.


There is a common theme in all these lessons, all the way from the evolution of matter, to life, intelligence, etc. and that is that at every stage we will see systems that form a new stable set of objects. These objects become a substrate for the formation of new, completely different objects which in turn will become a new substrate for new objects and phenomena.

We have seen how the chemistry of early Earth produced the set of objects (nucleotides, aminoacids, vesicles, metabolic reactions) that formed the next “stable” object: LUCA. Now it’s LUCA’s turn to populate the earth with the new “substrate” of organisms, from which new biological systems could be formed.


The Evolution of Luca (Last Universal Common Ancestor)

Within about half a billion years some form of LUCA is thought to have begun to emerge. We don’t have any clear evidence, but carbon isotope signatures indicative of some likely life processes was found in rocks 3.95 billion years of age. In the absence of fossil records that will become available much later on, we can only rely on “signatures” of biological activity present in rocks and atmosphere, much like we are trying to do these days in our search for life on Mars. We also have indications that about 3.5 Gya (Billion years ago). The first two of the three dominions of the “tree of life” split apart. They were Bacteria and Archaea. The third dominion, Eukarya, probably emerged from Archaea about 1 billion years later. To make it easier to grasp what happened in the course of evolution I am going to ask you to imagine that the 3.5 Gya milestone happened exactly 100 years ago. That means that every real Earth year would take a single second in our 100 years span.


The Main Branches of the “Tree of Life”: Bacteria, Archaea, Eukarya

It’s important to realize that we are not “more evolved” than any other form of life that exists today. We simply followed a different path and ended up as a “leaf” on the Eukarya tree dominion, while present-day bacteria are leaves on the Bacteria tree dominion. The Archaea dominion has been recognized only recently, as it was thought of as part of Bacteria, since they, like Bacteria, are single-cell organisms that do not have a defined nucleus in their cells (prokaryotes). The difference is in their internal chemistry (metabolism) that makes it possible for them to survive in “extreme” environments, like very hot springs and highly acidic environments. We, humans, emerged on the Eukaria side which is composed of cells that contain well-defined nuclei (eukaryotes). This property apparently facilitated the formation of multicellular organisms which evolved into plants, animals, and fungi. Note that single-cell eukaryotes do still exist, like amoebas and slime molds.

So, back to our representative “century”. 100 years ago (3.5 Gya) the first Bacteria (and Archaea) emerged. Note that the evolution of life is intimately connected with the evolution of Earth itself. Earth conditions allowed LUCA to emerge, and now photosynthetic bacteria begin to release oxygen into the atmosphere. The presence of oxygen will in turn favor the evolution or life forms that will take advantage of that (as we do now). Still “not much” happened for a whole 77 years when eukaryotic cells emerged. And even then it took another 7 years before multicellular organisms began to emerge. That’s only 16 “years ago” (555 Million years). So life on earth has been exclusively monocellular for most of the time.


The Evolution of Multicellular Organisms

Finally, multicellular organisms began to evolve. 14 years ago (500 Mya) vertebrate and invertebrate creatures were present in the oceans. 2 years later (420 mya) land plants began to evolve and create new habitats on the surface. 10 years ago (360 Mya) four-legged vertebrates began to populate the Earth surface, followed by dinosaurs and mammals 4 years ago (225 Mya). Around the same time the large continent that comprised most of the land mass (Pangea) began to break up and form present-day continents. Note also that between the arrival of four-legged vertebrates on land and the emergence of dinosaurs and mammals, 7 years ago (248 Mya) we had the worst mass extinction of life. 90% of ocean life and 70% of land life perished, and we are still not sure why, but as often happens with mass extinctions (and there have been 6 major ones) new niches are created where life can thrive, and we experience “explosions” in new species diversity.


Finally Homo Sapiens!

The second largest mass extinction happened about 2 years ago (65 mya). That’s the best-known one because it wiped out non-avian dinosaurs and it’s widely accepted to have been caused by the impact of a large asteroid. Fortunately for us, mammals survived, and, finally, an early hominid named “Lucy” appeared in Africa. That’s equivalent to about 41 days ago (4 mya). It took a while longer to see evidence of present-day anatomical humans and that was about 32 hours ago (130 thousand years ago) and about half as long to see evidence of human consciousness in the form of cave graffiti. So here we are, Homo Sapiens, just about a day old.

To recap, If we go back and look at life’s evolution as taking a hypothetical 100 years, it took about 30 years for a molecular substrate to give rise to LUCA and the first unicellular organisms. Life remained happily unicellular for about 84 years. Multicellular organisms evolved to more and more complex forms and it took another 9 years for mammals to emerge, and yet another 7 years to get to us. Our history started about a day ago. In just one day out of 100 years of life’s history we went from cave paintings to landing on the moon. Note that most of life on Earth is still unicellular! We are merely a leaf in a branch of the tree of life. How did we move so fast to become the dominant species on Earth? Are we about to become an interplanetary species? We will address these topics in the next lessons.


Recommended book

Why Evolution Is True by Jerry A. Coyne


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