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Evolution: Evidence

What is the Evidence for the Theory?

Similar structure: Bacteria, Mitochondria and Plastids have a double membrane envelope surrounding cytoplasm that contains DNA (in a loop in bacteria, and linear chromosomes in Eukaryotes), and many ribosomes (weight of 70S in bacteria, 80S in Eukaryotes).

Similar ultrastructure: Bacteria, Mitochondria and Plastids lack a cytoskeleton. Eukaryotes have a cytoskeleton consisting of microfibrils made of actin, microtubules made of tubulin, and intermediate filaments.

DNA sequence evidence: some gene sequences of Bacteria, Mitochondria and Plastids are similar, and dissimilar to most Eukaryotic genes. DNA of Bacteria, Mitochondria and Plastids lacks introns. A high level of DNA sequence homology exists between modern-day cyanobacteria and chloroplasts, and modern-day purple aerobic bacteria and mitochondria.

DNA processing: Bacteria, Mitochondria and Plastids synthesise DNA, RNA and proteins in the same manner. RNA is processed immediately in Prokaryotes, delayed in Eukaryotes.

 View animation of a processing prokaryote (12 Kb).

 View animation of a processing eukaryote (8 Kb).

Function of membranes: Bacteria and Mitochondria have two membranes: the outer is readily permeable, the inner impermeable to ions (allows generation of an electrochemical gradient, essential for formation of ATP). Chloroplasts are similar, though the inner membrane is selectively permeable. The chloroplast also contains a third inner thylakoid membrane which houses the proteins that enable light harvesting. Some enzymes associated with mitochondrial function are located in the outer membrane of the bacterial cell.

Mode of asexual reproduction: Bacteria, Mitochondria and Plastids divide by fission.

Endosymbiosis is still going on: some modern Protist cells engulf entire cells of different origin, and the association is stable in artificial culture. Some photosynthetic single celled Algae have an endosymbiotic eukaryote that itself contains a chloroplast. Free-living algae form symbioses with corals, resulting in the corals having the capacity to photosynthesise.  Global warming is causing corals to bleach (algae are shed). Free living protists often incorporate bacteria or yeasts. The symbionts provide a source of specific enzymes, vitamins or digestive capacity. Endosymbiosis is still going on.

Ancient history: fossils of bacteria are far older (3.5 billion years) than those for eukaryotic cells (0.6 billion years), though most fossils have been lost from the record because cells are readily degraded or not ever preserved. They are also so small that we cannot be certain that we are seeing cells at all.

Evidence Against the Theory

Location of genes for the function of the endosymbiont: Nuclear genes are necessary for chloroplasts and mitochondria to function. Proteins are transported from the cytoplasm to the endosymbiont, where they act with proteins formed from the DNA of the organelle.

However, DNA can be carried with transposable elements from cell to cell, thus providing a potential mechanism for movement from organelle to nucleus (we return to transposable genetic elements when we discuss reproduction in bacteria).

Absence of Mitochondria in some Protists: Some existing Eukaryotic Protists lack mitochondria. The organisms tend to be simple, and they may have hydrogenosomes enabling anaerobic respiration.

However, it is now believed that these organisms lost their endosymbionts as their structure became simpler. Indeed, the functions of these organelles is now supplied from the environment in which the cells are located.