The problem of sex (or why I'm not an atheist)
Nicolau - It seems to me that you actually are an atheist but that you're rather hesitant to accept the label, why would that be? Correct me if I'm wrong.
I will be an atheist the day irrefutable evidence leads me that way. When that day comes - and I suspect it will come eventually - I won't have any problems to take on the label. For the time being, apatheist, agnostic and anti-religion seem to fit with the evidence at my disposal.
I'm yet to read evidence of HOW organisms went from asexual reproduction to sexual
Eden I don't have time right now - and you should be doing your own research anyway - but sex as we know it today did not happen all at once. (potential for double-entendre is exceptional in this thread).
I think you are asking about how rather than why. Here is a brief summary.
For 2 billion years life was prokaryotic - bacteria and archaea. They don't have sex but they do swap genes around very promiscuously.
Eukaryotic cells - the stuff oak trees, yeast, rabbits and humans are made of - arose as a result of a single event around 2 billion years ago when a bacteria that could produce energy using oxygen found itself prospering inside an archaea that couldn't. As a consequence of this event complexity became possible for reasons I'm not going to describe right now. The bacteria became the ancestor of the mitochondria that we have hundreds of in every cell producing energy through aerobic respiration. Sex began in these early eukaryotic cells.
The early stages of sexual reproduction did not involve internal fertilisation or external genitalia or porn videos. All of that took a long time to evolve. The road to proper sex - as opposed to lateral gene transfer - has to do with the relationship between mitochondria - who still retain some of their own ancient genome - and their hosts who hold most of the DNA in a nucleus.
There are three basic elements to sex. Not the ones you are probably thinking about right now.
1. Cell fusion
Mitochondria benefited from cell fusion which gave them new hosts in which to multiply. With the loss of the prokaryotic cell wall fusion was simple. Jumping genes that originated in the bacterial symbionts would likely be the driving force to induce cell fusion just as they are in some simple eukaryotes today.
Meiosis begins with the doubling up of chromosomes before dividing into four daughter cells. Mitosis, simple cell division, also begins with doubling up of chromosomes. Only one key change is necessary to convert mitosis into meiosis - a failure to digest all the cohesion proteins or "glue" holding the chromosomes together. This confuses the cell into thinking it is primed for the next round of chromosome segregation before it has completed the first round. Voila - gametes!
All the machinery required to recombination was already present in bacteria. The precise method of recombination is identical in bacteria and eukaryotes. Bacteria take up packets of genes all the time from their environment and incorporate them into their genome. In this way they can maintain small genomes and acquire other genes when needed.
Once endosymbiosis had occurred sex was not only mechanistically simple it was inevitable.
There is a lot more I could tell you - like why and how two sexes - but that will hopefully give you a hint that the authors of your article are dullards who have no interest in discovering answers to questions that seem to support their superstitions.
For more detail see "The Vital Question" by Nick Lane chapter 6 and "Life Ascending" chapter 5 by the same author.
One of such reasons is the problem of sexual reproduction and the inability of evolution theory to provide a satisfactory explanation for its existence.
This sentence is only valid if you have not seriously studied evolution.
Cofty how tiring is it to have to go over this stuff like every day.
Cofty how tiring is it to have to go over this stuff like every day - ttdtt
I am starting to think there is a balance to be struck between being helpful and encouraging laziness.
Cofty how tiring is it to have to go over this stuff like every day.
Well, to be honest Cofty has been a relentless promoter of a certain worldview and we had a handful of ugly clashes in the past, but I have a tremendous amount of respect for him. We don't always agree (sometimes we don't even agree to disagree) but Cofty has been a lot more helpful to me and I'm sure others too that he may imagine. Only he knows if it's been a worthwhile endeavor or not. I'm showing here my appreciation.
Cofty, honest question as someone who knows not much of evolutionary biology but is trying to learn more daily but is still troubled in some redpects as OP is: How much of this explanation covering billions of years is based on observable evidence vs theory and assumptions and/or can any of those steps be replicated in a lab?
In other words, how certain is science regarding this process you describe?
...I have decided to lead my life where evidence takes me...
Exactly. I see choice there.
Eden... let me help you.... Nothing is more horrific to me then an ex JW that chooses Christianity... in my sight they have traded 7 existing men for one guy that might not have lived ,,, I mean that is sick
You have followed evidence much as I have but have not the same way.. when I saw evolution was true I also took an agnostic approach... I felt it reasonable.... However over time I learned it wasn't ...
Thus 2 years ago I became an atheist.... However that being said does not mean i;'m not spiritual... i need to understand the universe same as you... science will provide and answer even if not in our lifetime...
No asexual reproduction stems from eucriotic cells and is so far in the past it is not worth arguing about
how certain is science regarding this process you describe? - Sanchy
Its a great question. I think the most honest answer would assign a slightly different certainty value to different parts of the process. The age of life on earth is really certain as is the appearance of complex cells 2 billion years later.
The event that made complex cells - eukaryotic - possible was called endosymbiosis. This was when a bacteria became engulfed by an archaea. Lynn Margulis argued for this explanation for a long time before sufficient evidence proved it was true. It was a good example of how science works. It has now moved from hypothesis to "theory" in the scientific sense of the word. Unfortunately she continued to press her ideas to argue for "serial endosymbiosis" but the field of phylogenetics has proven her to be wrong on that.
The three steps to sexual reproduction is not really controversial. Each of them are very well known features of bacterial life. The genius was explaining the advent of meiosis from mitosis as described by Tom Cavalier-Smith.
All of these ideas are subjects of many scientific papers that you could access online. Thankfully writers like Nick Lane make the technical details more accessible to the rest of us via popular science books.
What I have described is the beginning of sex without sexes. It was a relatively simple operation that involved cells swapping genomes and mixing up their genes. In a world of high mutation rates and strong selection pressures this made it possible for useful combinations to "find each other" and others to be discarded from the gene pool.
One of the challenges to sexual reproduction resulted from the presence of mitochondria. These eventually gave up more than 99% of their genome to the nucleus of the cell but retained a few critical genes to do with controlling the rate of respiration. In this way they can respond quickly to changing demands of the cell on a local level. The problem is that the two genomes must work together very precisely. Mitochondrial DNA is not recombined, it is passed on through cell division in the same way as their bacterial ancestors.
It is for this reason that two sexes are the norm in sexually reproducing species. One larger gamete also includes the mitochondria while the other smaller more motile gamete contains just nuclear DNA. The former is by definition female and the latter male.
All the features of sex in multicellular organisms such as genitalia are much later inventions of evolution by gradual Darwinian evolution. Many of these stages of development can be observed in extant species. Consider the difference between mammalian sex and external fertilisation as practised by fish for example.
In modern complex multicellular beings like us most female gametes fail the test of the ability of the mitochondria to sync with nuclear genes for respiration. Female embryos have about 7 million oocytes by the fifth month of development. This number has reduced to around 2 million by birth. By the age of 40 there are only around 25,000 left. Only a few hundred of that original 7 million will ever mature and have a chance of reproduction. The majority of fertilised eggs spontaneously abort at a very early stage before the woman is even aware of being pregnant. It is thought that these are due to failures of cellular respiration.
Sex certainly makes successful reproduction much more difficult but far more advantageous for the cells that manage it. There are even a number of species that reproduce quickly via non-sexually but resort to sexual reproduction occasionally in response to availability of resources.