The transition from single celled to multicelled and sexual reproduction

[Phazzle]

Hello there. I am a non-scientist in search of some answers. I wholeheartedly accept evolution as science fact. Those more learned on the subject than I have assured me that it happened entirely apart from an intelligent designer and I take them at their word. So I am not "one of those," but I am having some trouble wrapping my head around a few particulars about evolution.

 

 

1) How exactly did single-celled organisms evolve into multi-celled organisms (complex organisms?)? I can't wrap my head around this question. Presumably there was a "first," complex organism, how did this creature make the transition if evolution is such a gradual process.

 

2) How did organisms evolve to reproduce sexually? That is how did natural selection favor sexual reproduction when there was, obviously, no precedent for it? Did a creature exist at one time that could reproduce both sexually and asexually? Do such creatures still exist?

 

 

Thank you in advance for your answers.

[Xittenn]

This isn't a simple answer question in any way. If I were to break this down into bullets it would look something like:

 

- amino acids mixed to form chains, RNA evolves

 

- viral like thingies, or subviral agents, float around doing not much of anything but keep organizing into new and more complex structures

 

- the lipid bilayer is formed and thus the cell membrane

 

- asexual reproduction evolves

 

- bacteria evolves, as does sexual reproduction

 

- eukaryote cells known as protists evolve because some bacteria engulf other bacteria and form a symbiotic relationship; these engulfed cells become organelles like mitochondria and chloroplasts

 

- slime molds, various species of protists, cover the world

 

- other forms of protists develop into alga

 

- alga take on an organized structure that resembles tissue specialization but is classified as multicellular colonial

 

- alga reproduce by sporangia that produce zoospores by meiosis, gametes that in turn either produce an egg (female gamete) or sperm (male gamete), which when the sperm fertilizes the egg a zygote is formed . . . .

 

- alga in shallow waters adapt to withstand dessication, terrestrial migration occurs

 

- cell specialization results in tissue and organ formation; true multicellular organisms evolve

 

- plants evolve two modes of reproduction, classification angiosperm and classification gymnosperms; angiosperms are flowering plants, where gymnosperms are like pine cones and variants; each has its own sexually reproductive gametes that specialize or adapt to better the survival of the species and so on . . . .

 

- fungi develop from another form of protist and so do animals; these two share a common ancestor . . .

 

 

I really hope this isn't homework, the mods might have my head . . on a platter . . they might anyway, some of the things I've stated as fact might be debated by many or all or anybody but myself, I'm pretty content with the outline . . .

[Phazzle]

No, this is not homework, your safe! Thanks for the response.

 

Much of it is over my head but it is less hazy now.

 

Thanks again!

[Moontanman]

I think it should be pointed out that even though early bacteria like organisms did not reproduce in a way that we would call sexual they did indeed exchange DNA among themselves, in fact bacteria do this today, individual bacteria of the same species and even across what we call species lines.

[Bio freck]

Lets go more inside to your question. First think what is "life" then comes your question about division of a cell. Just think you have all the requirements for a cell but still you can't produce the"protoplasm",the physical basis of life.What would you do???

The answer to your question is already answered finely by others.I would like to add some more.A cell divides to form a blastula (a mass of 16 to 32 cells,after sexual reproduction) and then it produces to form a gastrula (responsible for producing germ layers) to the foetus,organogenesis,parturition.

 

 

 

Note: This is about mammalian reproduction(human)

[Moontanman]

Lets go more inside to your question. First think what is "life" then comes your question about division of a cell. Just think you have all the requirements for a cell but still you can't produce the"protoplasm",the physical basis of life.What would you do???

 

What? Protoplasm is the physical basis of life, it can't be produced? Please clarify this statement..

 

The answer to your question is already answered finely by others.I would like to add some more.A cell divides to form a blastula (a mass of 16 to 32 cells,after sexual reproduction) and then it produces to form a gastrula (responsible for producing germ layers) to the foetus,organogenesis,parturition.

 

 

 

Note: This is about mammalian reproduction(human)

 

It is also off topic i think...

[Phazzle]

Thanks for your answers. I'll still admit that it is over my head so I'll ask a related question. Are scientists clear on how single-cell life forms evolved into the life forms that we are familiar with today? Is the scientific community basically in agreement on how this happened? Is there evidence for this?

[CharonY]

A limited form of transient multicellularity was most likely first developed in bacteria. An example of current bacteria is found in Myxococcus xanthus during fruiting body formation. It basically predates the formation of eukaryotes.

 

Eukaryotes most likely arose from a fusion of an archaeon and a bacterium, so in a way they could be considered multicellular. However, individuality apparently got lost to a large extent so that one usually would refer to them as unicellular.

 

Subsequently multicelluar eukaryotes developed (again most likely first with a transient, cooperative stage), still reproducing asexually.

 

Sexual reproduction arose most likely later and is still a bit of a mystery. My favorite model assumes that the origins are found in mobile genetic elements (but that would take quite a bit to explain it in a clear way).

Edited March 23, 2012 by CharonY

[Phazzle]

CharonY,

 

Thanks for breaking it down for me. Appreciated!

[Moontanman]

Thanks for your answers. I'll still admit that it is over my head so I'll ask a related question. Are scientists clear on how single-cell life forms evolved into the life forms that we are familiar with today? Is the scientific community basically in agreement on how this happened? Is there evidence for this?

 

 

http://science.slashdot.org/story/12/01/18/0118213/multicellular-life-evolves-in-months-in-a-lab

[Phazzle]

Thank you! That is so amazing. Very much above and beyond. I feel quite satisfied with all of the answers that you have provided. Let me just say that the work that you guys do is just amazing! There is a joke that the comedian Joe Rogan tells you should all check it out. It is exactly how I feel at this moment.

 

 

 

Thanks again!!!

[Xittenn]

Eukaryotes most likely arose from a fusion of an archaeon and a bacterium, so in a way they could be considered multicellular.

 

If mitochondria are assumed to be alpha proteobacterium then what came before this that would have been considered archaea? Also you seem to feel adversely about directing comments about sexual reproduction towards bacterium, may I inquire as to why?

[CharonY]

If mitochondria are assumed to be alpha proteobacterium then what came before this that would have been considered archaea?

I am not sure what you mean. Archaea are a completely different domain, distinct from bacteria. The evolutionary relationship is a bit tricky but it is a common misconception (presumably due to the name) that Archaea came before the bacteria. AFAIK the competing hypotheses are that archaea and bacteria share a common ancestor, another that archaea rose from bacteria.

 

lso you seem to feel adversely about directing comments about sexual reproduction towards bacterium, may I inquire as to why?

Bacteria have sex, but they do not use it for reproduction.

[Xittenn]

I am not sure what you mean. Archaea are a completely different domain, distinct from bacteria. The evolutionary relationship is a bit tricky but it is a common misconception (presumably due to the name) that Archaea came before the bacteria. AFAIK the competing hypotheses are that archaea and bacteria share a common ancestor, another that archaea rose from bacteria.

 

 

I'm saying the popular running theory--AFAIK--is that mitochondria were bacteria and not archaea. You are saying that the engulfed organism was archaean, so I'm asking you why you are making this statement? If I am missing something I can gain some insight into the matter. My intended meaning in my question was, if mitochondria are bacteria, then what organelles were archaean that came before this event? Or are you suggesting that mitochondria are assumed archaean? My notes come from Campbell's biology--sorry for any confusion, I seem to be completely incapable of transmitting a complete thought . . . . .

 

 

Bacteria have sex, but they do not use it for reproduction.

 

Weird! Very interesting, but very weird. Thanks for pointing that out!

[CharonY]

I'm saying the popular running theory--AFAIK--is that mitochondria were bacteria and not archaea. You are saying that the engulfed organism was archaean, so I'm asking you why you are making this statement?

 

You misunderstood something. Mitochondria are almost certainly bacteria. However, molecular evidence shows that early eukaryotes arose from fusion of archaea and bacteria. Not the mitochondrium- the whole eukaryote!

There are different hypotheses, but they have in common that presumably an archeaon underwent a symbiotic relationship with a bacterium. Later a fusion of these cells occurred. Depending on whom you ask some may argue that the first result was a eukaryote-like cell with a kind of nucleus and that a second symbiotic event led to the formation of a mitochondrium, whereas others propose that everything went in a single symbiotic event.

 

Weird! Very interesting, but very weird. Thanks for pointing that out!

Quite the contrary, using sex for reproduction is, from an evolutionary point of view rather weird (due to the two-fold cost of reproduction).

[Xittenn]

Thanks for clarifying that for me!