In 1966 microbiologist Kwang Jeon was studying a population of amoebae in the lab when they began to die off unexpectedly. He noticed thousands of tiny dots in the cytoplasm of each individual which turned out to be a bacterial infection. Most of them weakened and died but surprisingly a small percentage recovered and seemed to be back to normal.
Professor Jeon began to keep careful records expecting to discover that a few of his collection had an immunity to the bacteria. On examination however Jeon observed that each of the healthy amoebae that had survived the epidemic still contained more than 40,000 of the bacteria living inside them.
To study what was going on Jeon tried using antibiotics to kill the infection but as the bacteria died their host also died. Jeon and his team also tried surgically removing the nucleus from infected cells and transplanting them into another cell. They found that again the cell would die if it was transplanted into one without the bacteria. It had taken about 18 months - 200 cell generations - for the amoebae to become dependent on their former attackers.
Jeon eventually discovered that the bacteria had taken over the production of an enzyme that controls essential cellular functions such as cell growth and differentiation. In the following decades as genetic investigations became possible Jeon discovered that the bacteria and the host amoebae had actually exchanged some of their genes. By any possible definition this was more than a symbiotic relationship - the amoebae had become a different species with a radically altered genome. What Jeon and his team had observed was endosymbiosis in real time.
Creationists often falsely complain that evolution cannot be observed. This example and Lenski's long term E coli experiment show otherwise.
Kwang Jeon's work has implications for far more than just the evolution of one species of amoebae - it provides compelling evidence for the evolution of all complex cells.
For approximately 3 billion years - from 4 to 1 billion years ago - all life consisted of a relatively simple type of cell known as a prokaryote. These are usually round or rod-like with a rigid cell wall. Inside the cell there is very little to see, Their genome is minimal with the DNA arranged in a loop. Everything is streamlined for fast duplication. Given sufficient resources a single bacterium weighing a trillionth of a gram could found a population with a weight equivalent to that of planet earth in two days.
Every living thing you have ever seen from humans to oak trees are made up of more complex cells known as eukaryotes. These are significantly different from prokaryotes. By definition eukaryotes have a nucleus. This is the command centre of the cell where the DNA is kept behind a membrane. Unlike the single circular chromosome of prokaryotes the DNA of eukaryotes is arranged in a number of pairs of straight chromosomes. The genes come in chunks being divided by multiple stretches of random code. As a result many eukaryotes have massive genomes mostly made up of parasitic code. The record is held by the amoebae Amoeba dubia with 670 billion base pairs, 220 times as large as a human. On average eukaryotic cells are 10,000 to 100,000 times larger than prokaryotes although the two spectrums of size do overlap at the extremes.
Just as our bodies contain numerous organs, eukaryotic cells contain a variety of organelles outside of the nucleus. On average a cell contains a few hundred mitochondria that produce ATP, the fuel that powers all of the body's activity.
As early as the 1960s biologist Lynn Margulis proposed the hypothesis that mitochondria were originally free-living, oxygen-breathing bacteria that invaded anaerobic bacteria to their mutual benefit. For decades her ideas met robust opposition but the evidence continued to mount.
- Mitochondria closely resemble bacteria in size and shape. In particular they resemble purple-aerobic bacteria.
- They both use oxygen in the production of ATP, and they both do this by using the Kreb’s Cycle.
- Eukaryotic cells are incapable of producing new mitochondria which replicate by fission just like bacteria not by mitosis like eukaryotes.
- Mitochondria have a double membrane. The inner layer is very different from eukaryotes but has the same chemical composition as prokaryotes.
- Some types of antibiotics that kill bacteria also inhibit function of mitochondria.
- Margulis predicted that if her idea was correct then it would be found that mitochondria have their own DNA. This has also been proven correct. The DNA of mitochondria and chloroplasts is different from that of the eukaryotic cell in which they are found. Just like Kwang Jeon's amoebae gene transfer has occurred between mitochondria and the cell nucleus. More than 99% of genes have been given up by mitochondria with the exception a few genes that are essential to respiration so that power production in a cell can fluctuate to meet immediate demand.
The first appearance of eukaryotic cells is dated to approximately 1.5 billion years ago following a period when earth's oxygen levels rose significantly. The energy production of complex cells set the scene for an amazing radiation of different complex life forms. Bacteria "breathe" through their membrane which puts a practical limit on their size. Hundreds of mitochondria all contributing energy within the same eukaryotic cell opens up new possibilities for evolution and natural selection to explore.
Eukaryotic cells in plants have organelles called chloroplasts that convert energy from the sun into sugars. They too demonstrate all the same sort of evidence that they were once free-living cyanobacteria that merged with another prokaryotic cell through endosymbiosis. There is an interesting example of an intermediate stage of this process in the amoeba Paramecium bursaria that swims around pond water. It swallows photosynthetic green algae but doesn't digest them. When is swims into the light the algae produce sugar which both cells share on the go. It even shares food when they are in darker places where photosynthesis can't take place.
Paramecium bursaria filled with Zoochlorella cells
The hard work done by Margulis and others on endosymbiosis is an excellent example of how science works. A radical new hypothesis was rightly met with considerable skepticism. It was only after a long and difficult challenge over many decades when the evidence became irrefutable, that is was accepted as a valid scientific theory.
There are lots more details that could be shared on this topic but hopefully this provides a hint of the amazing results of scientific research. Like all creationist arguments the challenge of how complex cells arose is simply a failure of imagination - an argument from personal incredulity.
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#36 Mass Extinctions
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