Life’s Evolutionary Transitions!
When looking at the fossil record and trying to list the most significant events in our evolutionary history, you might want to list the earliest known fish species able to survive at least temporarily on dry land, you might also want to list the first known ape fossils which just like us appear to have walked on two legs.
While these fossils do represent fascinating transitions into new lifestyles they don’t represent large jumps in complexity the differences between two-legged and four-legged creatures mainly come down to slight adjustments and how joints fit together to see a truly large evolutionary jump what researchers call a major evolutionary transition we have to travel back much further in the fossil record.
In the late 1830s anatomist Theodor Schwann formalized the idea that the human body instead of being a single living thing is actually a collection of individual living cells. Today we tend to shrug this off as common knowledge, but think how amazing this is you are a colony. The earliest multi-celled animals found so far in the fossil record were primitive sponge-like creatures living in the sea. Before them all living things we know of were single-celled and came in three different types, Bacteria and Archaea which both tend to be extremely small, and then we have Eukaryotes most of which are still microscopic but far larger and much more complex than the others.
The cells that made up the first sea sponges and the cells that make up your body today are strikingly similar to single-celled eukaryotes the main difference is that your cells can no longer survive on their own, instead trillions of them are all working together in near-perfect harmony to make you. the move from single-celled creatures to multi-celled animals was a major evolutionary transition as a result your body now represents two distinct layers of life you as a whole and you as a colony bizarre as this fact is are stranger things.
Inside the cells of all eukaryotes single-celled and multi-celled, there are tiny structures called mitochondria scientists used to think these structures were simply body parts of the larger cell. In the late 1960s however, Lynn Margulis piecing together the discoveries of many others demonstrated beyond a reasonable doubt that mitochondria were once free-living bacteria. Mitochondria have their own genes they live, reproduce, and even die on their own inside their larger hosts. Mitochondria feed on the nutrients our cells produce, but they are not parasites in return for the food they consume they build and secrete special molecules called ATP molecules that our cells use as a source of energy kill the mitochondria and the larger cell will also die. This discovery means that you are not just two layers of life but three you as a whole you as a colony of cooperating cells and each of your cells as an interdependent community of its own profound as that is there’s more.
Inside each of your cells in each of your mitochondria, there is a genome, a large collection of genes that together allow yourselves to grow, function and reproduce. In the early 1970s, Theodore Diener was investigating the cause of illness in potato plants. The culprits he discovered were the smallest simplest reproducing structures ever to be described he called them viroids. They are not cells, they are not even viruses instead they are single free-living genes. Moving from plant to plant by hitching a ride on insects, farming equipment or even wind Viroids use the chemistry and nutrients inside plant cells to reproduce. The existence of free-living genes strongly suggests that the genome of the first cells on Earth and by relation the genome in your cells today can actually be thought of as a collection of individual genes working together in cooperation, in other words, your body represents four layers of life you, your colony of cells, the many communities making up each one of your cells and collection of genes that make up every cell’s genome. Each new layer of life is the result of what scientists call a major evolutionary transition. What was the cause of these transitions? the answer is, Cooperation. A major transition starts when free-living creatures team up to form a cooperative group, in the early stages of cooperation participants, are free to come and go as they please if a group sticks together long enough however division of labor will often evolve different participants begin specializing in different tasks as time goes individuals may become so specialized that they can no longer survive on their own. If the entire group becomes locked into cooperation depending fully on one another to survive and reproduce a new super organism has been forged and they made your evolutionary transition complete, from this point on the entire group will evolve together as one.
Models describing natural situations that might promote the evolution of major transitions have been put forth by scientists such as John Maynard Smith, Eors Szathmary, Stuart A. West, and W.D. Hamilton. Using these models researchers have been able to mimic natural scenarios in the lab, allowing us to directly witness the beginnings of major transitions evolved. In 1998 researchers set up a mini-ecosystem with small mouths protists and single-celled algae, the protists could easily swallow individual algal cells but had trouble eating cells that happen to stick together after reproducing in less than just 20 generations the algae evolved multicellular cooperation they form groups of eight tightly connected cells that could not be eaten by the protists. A similar experiment on single-celled yeast in 2011 showed that just 32 days after multi-celled colonies evolved clear division of labor also evolved giving rise to unique cell types specializing in different tasks. These two experiments show us how multi-celled organisms may have first evolved, but what about mitochondria and their permanent merger with eukaryotes. In a long-term study ending in 2008, a protist that normally eats bacteria was seen swallowing a species of algae apparently on accident that it was not able to digest inside the protists the algae was able to grow and reproduce when the protists reproduced as well both daughter cells contained algae after several years and many generations researchers found that when bacteria was scarce protists containing algae were much more likely to survive than those without, they avoided starvation by feeding off the waste products the algae produced.
This was the start of a brand-new relationship strikingly similar to what we find between our cells and the mitochondria that live inside. So to sum things up what caused life’s major evolutionary transitions? The answer is cooperation major transitions begin when a group of organisms join forces to better survive and reproduce if cooperation continues long enough a new super organism may emerge one that can then go on to reproduce and evolve as a whole and the pathway that led to animals along with humankind at least three major transitions have been identified resulting in four layers of life within your own body. Experiments in the laboratory have allowed us to directly witness the beginnings of major new transitions evolve.