About endosymbiosis

[tonyxon22]

The other day I heard in a documentary that the origin of the first eukaryote cell was due to an unique event in evolution called endosymbiosis, where the basic idea is that two prokaryote cells kind of fused and gave origin to a cell with differentiated organelles. There is a whole theory about this and what seems to be a very complete article about it in Wikipedia.

So my questions regarding this theory are:
1) The documentary mentioned that this was considered a singularity in evolution: it only happened one time and that all the rest of the eukaryote organisms come from this “primordial” cell or singularity. Is that true? How can that be demonstrated? Why couldn’t that happen in several places at the same time.
2) The article in Wikipedia says that estimations suggest that this event happened approximately 1.500 million years ago. In the other hand, life on Earth is believed to appear 3.500 million years ago. So with simple math I come to the conclusion that during the first 2.000 million years, all life in Earth was prokaryotes. Is this correct? How many different species of single-cell organisms existed back then, just before the endosymbiosis event? All the other diversity we see today has developed in only 40% of the time since life first began?
Thanks and best regards,

 

[Suraj M]

where the basic idea is that two prokaryote cells kind of fused and gave origin to a cell with differentiated organelles.

I don't know how to answer your question, but i'd like to add that there is a theory that says the now known mitochondria and chloroplasts used to be individual organisms. They somehow lost some genetic material that prevented them from regenerating/dividing, so they took the help of our ancestor, the prokaryote's nuclear material and became a part of it themselves, helping each other. I'm sorry but i don't have an answer to your questions.

 

[jim mcnamara]

It cannot be a once only happening. There are several type of organelles: mitchondria, chloroplasts to name tow important ones
So. Example: If heterotrophic single celled beasties came first, then how did we get plants with chloroplasts without another happening.
Or. If autotrophic beasties came first, how did they get a two-for-one deal? The short answer is there is no definite answer.

The likelihood is that it happened large number of times. As we speak maybe we can see the start of the process: zooxanthellae, a species of algae live inside the cells of reef-building coral. Salamanders have live-in boarders as well:

http://news.bbc.co.uk/earth/hi/earth_news/newsid_9446000/9446530.stm

So to assume a-one-time-only scenario is not a great idea.

 

[Ygggdrasil]

Although different endosymbiotic events clearly led to mitochondria and chloroplasts, it is an interesting question to determine whether all eukaryotic mitochondria derive from a common ancestor (indicating a singular origin of mitochondria) or whether various lineages of eukaryotes acquired mitochondria independently. The way to distinguish between these two hypotheses is to examine the DNA sequences from mitochondria (mtDNA) across eukaryotes and see whether they are all more similar to each other than they are to any prokaryotic genomes, or whether there some of the mtDNA sequences are different enough from other mtDNA sequences to suggest that they represent independent endosymbiotic events. The current evidence strongly supports a single origin of mitochondria (http://www.sciencemag.org/content/283/5407/1476.long).

This does not necessarily mean that only once in the history of life has a prokaryote colonized another cell to lead to a mitochondrion-like structure, but that only one such lineage has survived to the present day.

 

[Ygggdrasil]

2) The article in Wikipedia says that estimations suggest that this event happened approximately 1.500 million years ago. In the other hand, life on Earth is believed to appear 3.500 million years ago. So with simple math I come to the conclusion that during the first 2.000 million years, all life in Earth was prokaryotes. Is this correct? How many different species of single-cell organisms existed back then, just before the endosymbiosis event? All the other diversity we see today has developed in only 40% of the time since life first began?

To address your second question, it's worth remembering that Earth's environment has changed dramatically throughout its history. For example, the early Earth did not have an oxygen atmosphere until after the evolution of photosynthetic organisms like cyanobacteria. According to Wikipedia, oxygen did not begin to accumulate on Earth until about 2.5-1.9 billion years ago (http://en.m.wikipedia.org/wiki/Geological_history_of_oxygen). Remember that a key feature of eukaryotes is the presence of mitochondria, a structure evolved to facilitate aerobic respiration. Again, according to Wikipedia, eukaryotes arose 2.1-1.6 billion years ago (http://en.m.wikipedia.org/wiki/Eukaryote), around the same time as oxygen began to become prevalent on Earth. Thus, from this perspective, eukaryotes arose pretty soon after conditions became favorable for their evolution.

 

[Guco]

1) The documentary mentioned that this was considered a singularity in evolution: it only happened one time and that all the rest of the eukaryote organisms come from this “primordial” cell or singularity. Is that true? How can that be demonstrated? Why couldn’t that happen in several places at the same time.

Those arguments tend to rely on statistical methods. Keep in mind that the following two sentences are not the same:
- all current eukaryotic life derives from a single ancestor which resulted from endosymbiosis between two lifeforms
- endosymbiosis between life forms, resulting in an eukaryote, has happened only once

The same is true for the origin of life on Earth or the origin of the Y-chromosome.

2) The article in Wikipedia says that estimations suggest that this event happened approximately 1.500 million years ago. In the other hand, life on Earth is believed to appear 3.500 million years ago. So with simple math I come to the conclusion that during the first 2.000 million years, all life in Earth was prokaryotes. Is this correct? How many different species of single-cell organisms existed back then, just before the endosymbiosis event? All the other diversity we see today has developed in only 40% of the time since life first began?

Prokaryotes consists Bacteria and Archaea, which are considered two separate groups. There is massive diversity within these groups. We tend to equate increasing size to increasing complexity, but that's just one way of looking at it. For example, try looking up all the different methods life on Earth acquires energy, which is really what life is all about, and compare prokaryotes to eukaryotes. Or compare the total genomic diversity of the bacteria and archaea living on and in your body and your own genome.