Hi ExB.
Let me start by making a distinction between evolution and abiogenesis.
We know for absolute certainty that every living thing on earth from humans to haddocks descended from a common ancestor. Natural selection was one of the driving forces in creating the marvellous variety of design we see in the natural world. By design we mean the correlation between form and function. This was the genius of Darwin. He was not the first to propose evolution or common ancestry but he worked out how design arises without a designer - Alfred Wallace came to the same conclusion around the same time but he lacked the rigorous evidence that Darwin had accumulated over decades so they co-published.
Darwin's contribution has stood the test of time but he also got some things wrong. He also lacked knowledge of genetics - the means of inheritance and mutation.
Since Darwin's time multiple fields of science have confirmed the basic fact of common ancestry with astonishing detail. We have thousands of fossils showing the gradual change over millions of years in multiple lineages. One of the most complete lines shows the development of limbs and lungs as fish moved from sea to land. We also have numerous fossils of our own ancestral cousins - please note humans did NOT evolve from monkeys! Humans and primates share a common ancestor. Donald Prothero wrote a great book a few years ago outlining the fossil evidence - there have been so many exciting discoveries in recent years that when he came to update the text for a new edition he ended up writing a whole new book instead.
The most compelling evidence however comes from genetics. The proof of our primate past is of the same sort that is used every day to convict murderers and rapists and settle paternity disputes.
I look forward to discussing some of the specifics with you.
Abiogenesis is a different subject - it is about the origin of life itself, how geochemistry became biochemistry.
There has been a lot of progress in this field in recent years but the final answers are not yet in. The key to the solution is bioenergetics. There is no point showing how all the building blocks of life can arise spontaneously - and they can - unless they process of life can be sustained. The most promising work is being done by Nick Lane and his team at UC London. Very long story short he has shown how hydrothermal vents provide the perfect environment for the origin of life. Every living thing without exception sustains itself with a version of the same process of 'proton power' - a proton gradient across a membrane. Perhaps you remember the Krebs cycle from school biology? Lane has shown how every one of the parts of that process - all the chemical steps - can arise in the alkaline vent environment. It is very complex!
Complex cells such as the kind humans, animals and plants are made of, did not arise for billions of years after more simple life as a result of an event known as endosymbiosis.
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.
Okay enough for now. I'm going to repost an index to previous threads on evidence for evolution that I have posted previously. I hope you find it helpful.
