The Origin of the Eukaryotic Cell
Life and Early Earth
Since the early 20th Century, scientists have been attempting to uncover the mechanism behind the development of organic molecules from inorganic substances. In the 1920s, Aleksandr Oparin and John Haldane separately hypothesized that early Earth was a “primordial soup” of various chemicals with a relatively oxygen-free atmosphere. This concept has been tested several times over the years but creating a cell in the laboratory setting has proven to be insurmountably difficult. What we know about the evolution of the first cells remains largely unexplained.
Even more challenging to understand was the origin of complex cells from simple ones. At one time, animal cells and plant cells were thought to be the two fundamental types of cells. Today, we know that is not the case. Instead, the most basic division of cells is between prokaryotes and eukaryotes. This raises a natural question, however: If generating a cell is so difficult, how is it possible thosetwodifferent kinds of cells were able to arise and flourish on Earth? Science found an answer inthe work of a biologist named Lynn Margulis.
Margulis, along with several other scientists in the early and middle 20th century, noticed that DNA was present not only in the nucleus of eukaryotic cells but also in the mitochondria and chloroplasts. Interestingly, the genes found in this DNA did not seem to function as those in the nucleus did. Furthermore, though eukaryotic cells were already loaded with their own ribosomes, there were even more ribosomes found withinthe mitochondria and chloroplasts. Finally, the organelles were surrounded by a double-membrane, a characteristic shared by the cell membrane but not by other organelles.
Explanations and Endosymbiosis
What was the significance of these unexpected observations? Margulis ultimately proposed the endosymbiosis hypothesis. This was the concept that eukaryotes arose from a combination of early anaerobic bacteriaand the smaller aerobic bacteria that invaded them. Unfortunately, the scientific community at the time held the belief that eukaryotes had arisen purely through random mutations. To explain her ideas, Margulis needed to identify some kind of selective pressure that would have prompted natural selection to occur.
As the hypothesis suggested, early Earth was probably an oxygen-poor environment. Until that point, life had adapted to these conditions, but soon there was a dramatic change in the environment. Approximately two billion years ago, some prokaryotes, perhaps early blue-green bacteria (cyanobacteria), evolved the ability to convert sunlight and carbon dioxide into chemical energy. The first photosynthetic organisms had arrived.
Because these bacteria had gained an adaptation shared by no other organisms, photosynthetic prokaryotes had found a niche. Their numbers grew exponentially and with them came a relatively sudden increase in atmospheric oxygen as a byproduct. The high concentrations of oxygen proved toxic to existing anaerobic bacteria, killing the vast majority of them.
Those that survived this so-called “oxygen holocaust” had done so only because of an adaptation of their own. As the atmosphere changed, some of the larger anaerobic bacteria were invaded by smaller bacteria. These smaller bacteria were somehow able to use oxygen to produce energy. This invasion killed even more anaerobes but not quite all of them. Some were able to survive despite the change and soon found themselves in a symbiotic relationship. Because the larger organelles now had a food source and the smaller organelles now had “hosts,” a mutually beneficial interaction had occurred.
In this way, what we now know as mitochondria likely became part of what would become modern eukaryotic cells via endosymbiosis. Similarly, some of the photosynthetic bacteria responsible for the oxygen holocaust probably also took part in endosymbiosis, leading to today’s chloroplasts. We still have simpleprokaryotic cells on this planet, but the relatively few early eukaryotic cells that survived the endosymbiotic “invasion” went on to flourish as well, giving us the many different varieties of complex plants, animals, fungi, and protozoa we know today.
Conclusions
Margulis’ hypothesis, now considered a theory, has been supported over the years by many different studies. University of Pennsylvania scientists identified parts of protozoa (small eukaryotes) that appear to have had their origin in much simpler algae. Researchers at the University of Tennessee showed that amoeba (simple eukaryotes) could be infected with a bacterial parasiteand over many generations would ultimately become reliant on the bacteria for survival. Most recently, some of the DNA in complex eukaryotic organisms was found to have come from bacteria known as spirochetes.
Lynn Margulis’ own research culminated in several books and journal articles. Her contributions toscientists’ understanding of the origin of life transformed our views of evolution and of how long life has existed on Earth. Before passing away in November 2011, she was elected to the National Academy of Sciences and received the National Medal of Science from President Bill Clinton in 1999.
Short Answer Questions: Use the article to assist you in answering each of the following questions. Please rephrase the question in your answer.
- Photosynthesis from early blue-green bacteria is considered to have changed the atmosphere. Write the correct balanced formula for photosynthesis below.
- Oxygen is often viewed as a beneficial molecule. According to the article, on early Earth, how was oxygen a harmful substance, and what chemical process caused oxygen levels to rise?
- In proposing her hypothesis, Dr. Margulis had to explain why endosymbiosis occurred according to the principles of evolution. What was the selective pressure that led to endosymbiosis?
- What evidence indicates that mitochondria and chloroplasts were once free-living organisms? Toexplainyour answer, include three cellular structures found in modern eukaryotes that suggest that they once were independent prokaryotes.
- According to the passage, small aerobic bacteria invaded larger anaerobic bacteria. Eventually, both cells existed in a symbiotic relationship that was beneficial to both. What is the name of this type of symbiotic relationship, and what benefits did each cell enjoy in their new form?