A History Of Life, Part III

Not so long after photosynthesis and cellular respiration evolved, organisms started to eat each other. Single cells don’t have mouths, so the only way that one cell might eat another is by physically engulfing it. As prokaryotes diversified and competition for limited resources increased, it made sense that some cells would want to start eating each other this way, because to do so would be a very efficient way to obtain a big package of organic molecules all at once. Odd as it seems, the evolution of eukaryotic cells may be largely the consequence of cells trying to eat each other and getting indigestion.

This idea is known as the endosymbiotic theory of eukaryotic evolution, and was first proposed by Lynn Margulis. The endosymbiotic theory suggests that at least two of the most important organelles found in eukaryotic cells originated when one prokaryote engulfed another, and instead of ingesting it, it developed a symbiotic relationship with it. The Greek root symbio means “living together”. Endo is “inside”.

To get to endosymbiosis, probably the first thing that had to happen was that the cell membrane of the original eukaryotic cells had to evolve to become more flexible. Once a cell has a flexible membrane, it would be able to fold its membrane around and engulf another cell. It would also be able to do other things, like invaginate itself so that it could make internal compartments. For example, an enfolding of a cell membrane is thought to have given rise to the nucleus of modern cells, by providing an internal compartment in which the DNA of the cell could be protected from other biochemical activities of the cell.

It’s not clearly exactly how the origin of the nucleus is related to the evolution of the eukaryotic genomes, but there is no doubt that the existence of the nucleus in modern cells is essential for the way in they manage their DNA.

A Win-Win Situation

Once cells had developed a flexible membrane, larger cells began to engulf smaller cells as a way to obtain resources. Lynn Margulis contended that it is possible that a small prokaryote occasionally became engulfed but failed to be broken down. The smaller engulfed cell would now be trapped inside the larger cell. Of course, this arrangement could only be maintained over time if both cells benefitted in some way from the arrangement. In other words, this arrangement had to be adaptive in some way to both cells.

The advantage to the cell that had been engulfed seems obvious. It is now living inside of a very nutrient-rich environment, much more so than the outside. What could be the advantage to the large cell, serving as a host to the smaller cell? Margulis argued that there would be an advantage if the cell that was engulfed happened to have a more adaptive set of methabolic pathways. There are two such pathways that had evolved: photosynthesis and cellular respiration.

Specifically, Margulis suggested that the evolutionary origin of the organelles called mitochondria, which are specialized for energy processing, occurred when a cell capable of cellular respiration became endosymbiotic with a larger cell that lacked these pathways. Cellular respiration is much more efficient in the way it extracts energy from the breakdown of organic molecules. These proto-mitochondria would benefit by having a buffer from the rest of the world and a constant supply of nutrients. In return, the engulfing cell gained energy that was produced by the engulfed cell.

Similarly, Margulis suggested that chloroplasts, which are organelles specialized in photosynthesis, originated when an early eukaryotic cell engulfed a smaller photosynthetic cyanobacteria. As natural selection acted to make the host cell and its endosymbionts more dependent, this confederation of cells would eventually be integrated into a single organism. This single organism was a eukaryotic cell.

The Evidence

Margulis was greeted with skepticism when presented her theory. Over the decades that followed, however, a growing body of evidence has accumulated to suggest that she had it exactly right. Something like this happened about 1.5 billion years ago. Much of the original evidence was circumstantial. For example, if you look at the structure of mitochondria or chloroplasts, you see that they have not one membrane surrounding them, but two of them. This is what you would predict if a cell had been engulfed and maintained its own membrane.

There’s also the interesting fact that mitochondria and chloroplasts have their own genomes. It turns out that they not only have their own genomes, but they replicate by cell division themselves. This means that when a eukaryotic cell divides, in advance of that, the mitochondria and the chloroplast themselves have to divide. When the eukaryotic cell divides, there’s enough chloroplast and mitochondria to go around. The eukaryotic cell itself does not replicate mitochondria and chloroplast, they replicate themselves.

The most interesting piece of evidence, though, has come from phylogenetic studies of the mitochondrial and chloroplast gene sequences. If you look at the structure of the gene sequences found in these organelles, you find that they resemble their presumed ancestors: cyanobacteria.

Another interesting thing is that they have evolved together with the eukaryotic cell. Although they carry their own genes, they don’t carry enough genes to live entirely on their own. The genetic function of a mitochondria is in part due to genes that it bears in its own genome, and in part to genes that are found in the eukaryotic cell’s genome found in the nucleus.

The last interesting twist in our understanding of the evolution of eukaryotic cells is to ask: Who were the progenitors? It turns out that genetic evidence suggests that the archaea, not the bacteria, gave rise to the engulfing cell that became the eukaryotic cell. The organelles that were engulfed, however, were bacteria. What that means is that after the initial division of life into two major lineages, the archaea and bacteria, there was a reintegration of those cells to form this chimera as an innovative complex cell. To be continued…

Theories of Origins Before Darwin

What were the theories of origins before Darwin? What was there before Darwin? What did people think about the origin of life and the different species on Earty? One thing characterizes people, as Descartes would say, they think. As thinking creatures, we have always wondered about how the universe and things in it originated. We are particularly interested in our own origin.

The first chapter of Genesis contains the Christian creation account. It tells of God creating the heaven and the Earth, plants and animals, and then man in God’s image. All in six days. The Bible doesn’t state when this creation occurred, but most early Christians probably assumed that this did not occur too long ago. In the 1600’s, the Anglican bishop James Ussher fixed the date of creation at 4004 B.C.E. This is the established biblical view that continues to the present.

The Early Scientific Accounts of Origins

Over the past 2000 years, this creationist account did not exist alone within the western tradition. Religious accounts of origins, at least for the past 2000 years, have competed with scientific accounts of origins. Science began with the Greeks, about 600 B.C.E. At this time we find the firsts scientific explanations for natural phenomena.

Although many Greeks retained religious theories about nature founded on revelation or mythical stories, some philosophers proposed materialistic explanations founded on reason. What do I mean by materialistic? This means that they explained natural phenomena without recourse to God or the supernatural. These philosophers said that natural phenomena can be explained as the result of physical matter moving in accord with natural law, with God, at most, as the remote creator of the primordial matter and the laws of motion. We find this sort of account in Plato, for whom God created the primordial matter and its laws, and then left it operate.

Biological origins posed a particular puzzle for Greeks who tried to devise purely materialistic explanations for natural phenomena. Biological organisms, people specially, seemed much more intricate and intelligently designed than just rocks or mountains. They seemed created, and creation implies a creator.

So, to explain the origin of biological organisms, early natural philosophers, like Anaximander and the so-called atomists, proposed crude theories of evolution. They are not very detailed, but they had the idea that there was some sort of spontaneous generation of life and somehow species could evolve over time. They weren’t worked out very well.

Aristotle critiqued these ideas. Aristotle himself was an atheist, and first and foremost, a biologist. He was a very avid observer of life, particularly of fishes. Based on his close study of animals, Aristotle defined a species as a breeding group. A group of particular animals or plants that can breed, and produce offspring that eventually could reproduce. He concluded that species were fixed.

Rejecting both creation and evolution, Aristotle simply saw the species as eternal. They always existed. Later Christian philosophers tried to integrate Genesis with Aristotle. They typically viewed each species as created by God in the beginning, but then, using Aristotelian authority, asserted that these species remained fixed for all time in a perfect (albeit fallen) creation.

This was the dominant view for a millennium in the West. It began to break down, though, when religious authority began to break down.

Deist and Atheist Accounts

The breakdown of religious authority finally occurred during the Enlightenment, in the 1700’s. Notions of evolution began creeping back in. This happened particularly in France, where natural philosophers again struggled to devise purely materialistic explanations for life. Seeking to push God back to the beginning, deists proposed a variety of ideas. They proposed that the solar system was created not by God, but rather a comet once hit the sun and knocked off a bunch of matter, which separated, each piece becoming a planet.

They also proposed ideas for the origin of species. They said that the tremendous array of species evolved from a few common ancestral types. Some of the French natural philosophers were even more atheistical. Denis Diderot, for example, a committed materialist, proposed that all living forms developed by random chance mutations from spontaneously generated organisms.

Probably the most influential natural philosopher from this period was the astronomer Pierre Laplace, who proposed a purely materialistic explanation for the origin of the solar system. He said that the solar system was once a big rotating gas nebula, and as it rotated, centrifugal and centripetal forces would pull in matter to the center, which became the sun, but as it pulled in, it left little blobs of material that collapsed into the different planets. This was called the nebular hypothesis.

When Laplace described his theory to Napoleon, he was asked “how does God fit into it?”, Laplace famously responded “I have no need for God in my hypothesis”.

All the ideas that we’ve gone over were highly speculative and were driven more from philosophy than empirical scientific research. There were a few discoveries at the time, though, that reinforced these ideas.

For example, Abraham Trembley detected that polyps, which are very simple sea creatures, could regenerate. By cutting them into pieces they regenerated the whole. They could be flipped inside out and still operate. People saw this as “almost spontaneous generation”. Philosophers took this as scientific evidence for their speculations.

Overall, however, the empirical research during this period cut the other way. Even if these ideas were speculated about, when people actually did the experimental and observational work in nature, most of them opposed the evolutionary ideas. The generation after the Enlightenment reacted against the speculative nature of evolutionary ideas. They returned to creationism, although not the creationism of the Bible, but a creationism based on scientific evidence. I will discuss this in my next post, when we see how Georges Cuvier founded modern biology.

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