In Defense of Evolution

[ FOLLOW THE ORIGINAL THREAD HERE ]

Never have I so single-handedly tackled so great a debate as this....

Opening statement by Sevendarks:

do you guys believe in evolution? I certainly dont believe in Creationism, but after the reading the this article

http://www.taemag.com/issues/articleid.18132/article_detail.asp

I am starting to doubt the validity of evolution.

Its a long article, so I will spell out one of the larger points.

The fossil record doesnt show any transitional species. E.G. there are no fossils that show thebridge from one species to another. All the species are fully formed. no catdogs or snakebirds ect.

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Part I

Being a biologist, I am fully supportive of the evolution theory. Not exactly the one first put forth by Darwin (which, many evolutionists agree, has a couple of flaws - hell, in Darwin's time there was no such thing as genetics yet), but the refined one that we now know today.

Sevendarks wrote:
The fossil record doesnt show any transitional species. E.G. there are no fossils that show thebridge from one species to another. All the species are fully formed. no catdogs or snakebirds ect.

This is the statement often put forth by Intelligent Designers and Creationists as 'proof' that evolution is fallacy; however what they've said doesn't quite reflect the real situation, and moreover, isn't accurate. Let me explain.

The story of the theory of evolution began when Charles Darwin, while on an expedition, noticed that the beaks of some birds there had differently shaped beaks that better suited them for their environment (with respect to the obtaining of food). He sat down, mulled it over with a friend, and then came up with the natural selection theory that we've come to know today - the survival of the fittest.

In Darwin's day, however, genetics was something that remained to be discovered. Because genetics plays an important role in the discourse of evolution, I will attempt to explain genetics very briefly.

We now know that our inheritance is dictated by DNA. Or to be more specific, the genes in our DNA. Now, in any given population, there are variations of each trait or gene, known as alleles. In basic Mendelian genetics, there are two forms of alleles: dominant ones and recessive ones. Put two dominant alleles together, and you get a dominant trait. Put one dominant and one recessive, and you'd still get the dominant trait because the dominant allele overshadows the recessive. It's only when you put two recessive alleles together that the recessive allele will manifest. Take for instance, eye colour. Brown is a dominant eye colour; blue the recessive. Two blue-eyed parents can only produce blue-eyed children; brown-eyed parents can produce both brown-eyed and blue-eyed children if they are heterozygous (that is to say, if they each have one blue allele and one brown one). However, there are certain cases where Mendelian genetics do not apply, for instance blood grouping. This is because there are THREE alleles involved - A and B (which are equally dominant, called 'codominant') and O which is recessive. Putting AO and AA together will get you blood type A; BO and BB will get you blood type B; but because A and B are equally strong, what you get when you put them together is blood type AB.

Now that we have the basics down pat, the next topic is: how does genetic change occur? This can happen a few ways:

1. Mutations: this is all something we're quite familiar with, thanks to X-Men. Mutations are changes that occur in the DNA of an organism. The changes range from the very big to the very small, and may involve as little as one base pair, or as large as an entire chunk of genetic code, and involve different mechanics ranging from enzyme reading error to jumping genes to insertion of DNA chunks by, say viruses. Now, we have four different bases that make up our genetic code: adenine (A), cytosine (C), guanine (G) and thymine (T). Three of these bases put together makes one amino acid (the basic building blocks of protein). Now, what happens when one base accidentally gets switched? Because the amino acid code is degenerate, a replacement of the third base will usually be of no consequence since the amino acid that is being coded will still remain the same. However, replacement of the first or second will usually result in a different amino acid altogether - which can sometimes result in disaster. What is even more disastrous is a frameshift mutation, which involves the insertion or deletion of one base, or a few bases, which causes the entire genetic framework to shift drastically, so that everything downstream of the mutation is changed. (Ghastly, isn't it) And then of course you have Barbara McClintock's famous 'jumping genes', which is exactly what its name implies: certain chunks of DNA on one arm of a chromosome could be transposed to another region of a chromosome, causing genetic rearrangement. The shuffling around of DNA is what causes us, among other things, to have an incredible repertoire of antibodies; moreover it shows that DNA is far more plastic, fluid and changing than was previously thought.

2. Gene flow: when members of one population commingle and breed with members of another population (within the same species of course) whose genetic traits are subtly different, this creates hybrids and adds new alleles to the pool, thus increasing genetic diversity; furthermore, it may also tip the balance of allele frequencies in the population.

3. Genetic drift: this refers to the seemingly random flux of allele frequencies in a population. In a relatively small population, this can become devastating: an allele can become fixed or eliminated altogether over generations.

Have I lost my audience already? I'm sorry!

Now that we've nailed down the basics of genetics, let us return to our original topic: why are there no transitional fossils?

What are transitional fossils in the first place?

They are fossils that show an intermediate stage between two 'distinct' species - that is to say, a specimen exhibiting characteristics that are a 'hybrid' of the species before it and the one after, that is supposedly indicative of the evolution of Species X to Species Y.

Unfortunately, the whole 'transitional fossils' thing has been utterly miscontrued by Creationists and those who believe in Intelligent Design (would anyone like an explanation on ID?). I shall attempt to outline the argument against evolution based on 'transitional fossils' and explain against it:

1. There are no transitional fossils; every species appeared fully-formed

Oh but there *are* plenty of transitional fossils. (and we aren't even talking about the more controversial ones) IDers and Creationists have simply chosen to ignore them. We have a fairly extensive family tree of the human species from Australopithecus anamensis (which diverged from Ardipethecus ramidus about four million years ago) to Homo habilis to Homo heidelbergensis, finally leading to Homo sapiens. (So far there are about 17 members in this family tree detailing the evolution of humans and their kin, and I've no doubt they'll find more. If anyone wants the entire tree, PM me). Evolution of horses? Plenty of fossil records, all the way from Hyracotherium way back 55 million years ago, to our modern Equus. (and whose family tree is even bushier than the Homo's) Reptiles to birds? Everybody knows of dinosaurs and birds, and their intermediate Archaeopteryx.

2. A transitional fossil is not proof of an evolutionary relationship because it cannot be proven that the fossil is an ancestor of any later organism

Well, barring the invention of a time machine that will enable us to journey back and study evolution, they've got it right. A fossil record is only indicative of evolution - supporting evidence of the argument in favour of evolution, if you will. You're not going to get, 'This is irrefutable proof that this-and-this runs this way, BLAM!'; you get 'This-and-this shows that this theory holds.' There's no way for us to witness the birth of each extinct species to determine if A gave rise to B. And even when one fossil closely resembles another, there is the question of whether one descended from the other, or if the two diverged from a common ancestor.

If that is the case, why are these fossil records important? Because the mere appearance of such a 'side-branch' intermediary species infers that there were other similar creatures around at the time it existed, one of which would have lead to our species B. There was a time, for example, when we were taught in schools that we were descended from Neanderthals (Homo neanderthalensis), which in turn was descended from Homo habilisH. sapiens eventually drove the latter to extinction. (behold the detriment of man!). In many cases, here is no steady progression from species A to species B to species C all the way down to Z; in many cases several different species lived side-by-side. (Evolution is a bush, not a coconut tree; thus the arguments about 'snakedogs' is irrational) And if you were to draw up a phylogenetic tree, you'd discover that these transitionals fall within the area you expect them to, thus further supporting the evolution theory.

3. The current dating methods are inaccurate, and cannot be relied on for the placement of these so-called transitionals

Well, had we still been using the carbon-14 dating method, I would have to agree. But since the 1950s there has been the development of three new clocks for dating: the uranium series, the rubidium/strontium and the potassium/argon, the last of which is by far the most accurate and is able to give reliable dates even for young rocks (of a million years of age or less), given that it has a half-life of 1,300 Ma, as opposed to the 5,730 Ma of the carbon-14 clock. Not to mention it can reliably date very small samples. (sample sizes, not size of samples)

4. There are gaps in the transitional fossil record

Of course there are, you doint! What did you expect, a complete record of how the cranium size of this animal increased/decreased, millimeter by millimeter?

Seriously speaking, you don't have a gradual progression of fossils from species X to species Y, for a number of good reasons:

1. Palaeontology isn't over and done with yet. Palaeontologists haven't yet excavated *all* the fossils on the planet yet! Consider this: of all the fossils found so far, a measly 1% or so are of terrestrial vertebrates; at least 95% are of marine invertebrates. At any rate, hundreds of fossils are being found every year (only Europe and North America have been extensively combed over at this point). Gaps are still being filled - those in the human tree certainly are! And please bear in mind that not everybody who sets out to hunt for fossils will find valuable ones. Not everybody is Eugene Dubois, after all.
2. Evolution isn't all that gradual. Refer to the above genetics discourse. Mutation is random - it can be large-scale, or it can be barely noticeable. Evolution and mutation does not dictate that one species will evolve into another in precisely X steps, with a variation factor of Y in each step. In some cases, evolution can even run backwards - see the evolutionary tree of the horse for an example of this. Selective pressure merely governs the selection of the organism that is best suited for its environment; it doesn't necessarily specify which way the evolution process runs.
3. And then you have variations among members of the same species in the same population, within the same generation - given that fossilisation of carcasses is such a rare event (the conditions have to be just right; would anyone like an explanation) and therefore there aren't all that many that have survived to tell the tale of three billion years of evolution, what are the odds that you'll find an immediate link between one species and the next? There is no way you're going to dig up all the links to the chains.
4. There are stratigraphic gaps, which causes discontinuity in the fossil-bearing strata. You're not going to find happy continuous stratas running from Precambrian to Cambrian to Ordovician to Silurian to Devonian etc etc. For instance, there are no mid-Jurassic period fossils anywhere around the world; the Jurassic and Cretaceous period yielded only several mangled tetrapods!
   Moreover, it certainly doesn't help palaeontologists that fossils from very far back don't generally survive the crushing and folding and general tumult of the earth over the passage of eons.
5. Most fossils, alas, suffer from lack of media attention. Everybody wants to know about the human 'missing link'; but who wants to know what species linked poor Parahippus and Hipparion? What, you don't know what I'm referring to? My point exactly. Who wants to spend a day with me, raiding the Paleontology journals section at the library? I'm sure we'll find more links than we can digest.
6. The documentation of species-to-species transition takes blood, sweat and tears... and an incredible amount of time. Phillip Gingerich was among the first to tackle this arduous task; it took him one decade to even detail two lineages. It remains to be seen if some of these transitional 'gaps' are for real, or if nobody has gotten round to fitting them together yet.
7. Creationists are never satisfied. Provide them with a transitional fossil (say B) between species A and C, and they'll demand a transitional fossil between A and B. Provide them with that and they'll demand another transitional fossil between A and A'. When will it ever end? Creatures evolve, they don't bloody morph!

At any rate, consider this: When you get right down to it, every single species is a transitional one! Think about it. What is evolution, if not a bushy series of transitionals?

5. Even scientists don't agree with each other that evolution occurs

BEEP - WRONG! The debate between scientists isn't whether evolution occurs (they all know it does), but whether the evolution is progressive/gradual, or whether it's punctuated (as in, a couple of graduals, and then a big leap, and then a couple more graduals etc). It's just that IDers and Creationists have deliberately taken this argument out of context in an attempt to fuel their own flames.

To end the story: Living proof that evolution occurs

Few are lucky enough to witness the evolution of a new species. Kwang Jeon, a scientist at the University of Tennessee was one such person. I shall quote verbatim from Lynn Margulis and Dorion Sagan's 'Microcosmos':

Jeon had been raising and experimenting with amoebae for years when he welcomed a new batch to his laboratory. After putting the new batch into special small bowls next to other amoebae gathered from all around the world, he noticed the spreading of a severe illness. Healthy amoebae groew round and granulated. They refused to eat and failed to divide. Bowl by bowl, more and more amoebae died. The few that grew and divided at all did so reluctantly, about once a month instead of once every other day.

When Jeon examined the dead and dying forms under the microscope, he noticed tiny spots inside the cells. On closer inspection he saw that about 100,000 rod-shaped bacteria, brought in by the new amoebae, were present in each amoeba. The rod bacteria had infected the rest of his collection. Yet the disease did not prove a total catastrophe. A small minority of infected amoebae survived the scourge. These 'bacterised' amoebae were delicate, fragile organisms, over-sensitive to heat, cold and starvation. They were easily killed by antibiotics, which, while deadly to bacteria, did not harm his normal 'nonbacterised' amoebae. A change was occurring. The two types of organisms, bacteria and amoebae, were becoming one.

For some five years, Jeon nurtured the infected amoebae back to healthy by selecting the tougher ones and letting the others die. Still infected, the amoebae began to divide again at the normal rate of once every other day. Reproductively speaking, they were as adapted as their uninfected ancestors. They were not rid of their bacteria - they all harboured 'germs'. But they were cured of their disease. Each recovered amoeba contained about 40,000 bacteria.

For their part, the bacteria had dramatically adjusted their destructive tendencies in order to live inside other living cells. Thus, from a violent confrontation emerged a new symbiotic organism, bacterised amoebae. Now, some fifteen years after the plague, the permanently infected amoebae are no longer sick but alive and well and living in Knoxville, Tennessee.

But the story does not end here. Applying his expertise in manipulating amoebae nuclei, Jeon followed up on his original experiments. From friends, Jeon reclaimed some of the amoebae that he had sent off before the epidemic and which had never been exposed to the pathogenic bacteria. With a hooked glass needle, he then removed the nuclei from both infected and uninfected organisms and exchanged them.

The infected amoebae with new nuclei lived on indefinitely. But the 'clean' amoebae supplied with nuclei from cells that had been infected for years struggled for about four days and then died. It seemed as if the nuclei had become unable to cope witha 'healthy' cell. Had they actually come to need their bacterial infection?

To find out, Jeon prepared another batch and mounted a rescue. Just a day or so before the bacterialess amoebae with their new nuclei would have died, he injected some of them with a few bacteria. The bacteria rapidly increased to the level of about 40,000 per cell, and the sick amoebae returned to health. A symbiotic habit had been formed; the bacteria were the 'fix'.

Jeon's amoebae can be killed by penicillin, which binds to sites in the cell walls of the bacteria within them, destroying the interdependent population that is the cell. The pact between bacteria and amoebae has become so intimate and strong that death to one member of the alliance spells death for both.

I'm sure that there's stuff I've missed out, so I'll probably be back to add all those things. Till then...

- Far.

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Part II

Hello SevenDarks,

In reply to your questions:

Sevendarks wrote:

The horses you are talking about are the very small cloven hoofed ones correct? Well, I can understand how that horse evolved into the ones we have now. Look at the diversity between dogs(size and shape) and that is in a relatively short period of time. What I dont understand, and what I believe the author was talking about was the transition between say horses and cows(I realize that isnt the proper example but I am not a person familiar with this stuff)

This may surprise you - dogs are actually closer related to bears than they are to cats. (the dog-cat split occurred much earlier than the dog-bear one). The ancestor to dogs was Cynodictis, which was a dog-bear like creature. This gave rise (directly? indirectly?) to Hesperocyon during the Oligocene period, which subsequently split up into the lines of dogs and bears. Weasels split off sometime after the dog-cat one.

... Oh shit, you were talking about horses and cows. Sorry!

The lineage of horses and cows can be traced back to the Condylarths, which are primitive hoofed animals. Sometime between the Cretaceous period (end of the age of dinosaurs) and the Eocene, this line diverged into the line that would produce horses/tapirs/rhinos and the line producing cloven-hoofed animals. The camel and cow lines diverged sometime during the Eocene epoch itself (which happens to be the epoch where mammals became the dominant animals).

Archaeopteryx

That was the dinosaur with feathers and full bone structure(not hollow) and some sort of wing was it not? I can understand how evolution explains that. I can understand how some lizards + snakes evolved flaps of skin to glide. What I dont get is how one species can go to another. What did it mate with during this stage? I dont get it, and I dont really understand enough, but I do find it interesting.

A good question, Darks. I'll address the mating part first. Genetics dictates that it is not possible for members of different species to mate and produce fertile offspring - that is to say, offspring that are capable of reproduction. Sure, these matings can occur - how else can you get mules and zebus? - but because of the difference in the number of chromosomes, these creatures are sterile. (and if a human, with 46 chromosomes, were to mate with an ape, with 48 chromosomes, there *is* a chance this will produce offspring, but 23+24=47 chromosomes. One of the chromosomes will be without a partner, and thankfully because of all this - how do you divide 47 by 2 during meiosis? -, we will not have a planet of the half-apes anytime soon) Which means that the only things that any said species can mate with are other members of its species.

Having said that, perhaps you might understand the process of evolution if I were to use an allegory of sorts. Imagine a small creature with four limbs that lives on trees. It would probably have claws of some sort to facilitate movement up and down branches. Because it is herbivorous and smaller than many other animals, it is prey to bigger carnivore. A variant creature who had stronger limbs might be able to escape becoming dinner to a ground-bound carnivore - but remember, the creature will probably be living quite high up on the tree. Falling out of the tree would probably be fatal to it. Any mutant of the creature that happened to have, say, looser skin on the sides of its body that connects with the ends of its forelegs, would have a slightly greater area-to-mass ratio than its 'normal' cousins. Now, taking into account that the greater your surface area in relation to your mass, the less likely you are to plummet to the ground like a stone, an animal that has its weight spread out over a wider area would fall less quickly than one whose weight is all packed up in a lump; therefore this mutant variant would be able to survive some falls that would incapacitate its normal cousins, by being able to glide a tiny bit.

If this bit of advantage causes this creature to be more successful to survive until it is old enough to mate (and assuming the mutation is carried in a gamete cell), selective pressure would cause this type of mutant to be selected over its normal cousins, so that over the generations you see a decline in the number of normal cousins and an increase in the number of gliders.

Now, imagine it doesn't stop there. Imagine that there are more mutations happening (as there will be, of course, in nature) with regard to this loose skin flap. With each mutation in favour of a glider, you will see the steady evolution of creatures that have only vestigial glider wing-skin to creatures that have the rudiments of wings to creatures that may or may not develop fully functional 'wings' that can be controlled by muscles. Because the forelegs would then be primarily used in controlling movement of these creatures in the air, they would gradually lose their function as climbing appendages. You would probably, over time, also notice a change in the posture of these creatures - instead of having all four limbs planted on the surface of a tree or branch, with its torso close to the air, our glider creatures would probably evolve (through selective pressure) to have stronger backlegs to help steady its landing; clinging onto a branch would depend more and more on these backlegs. If you were to take all this through the course of a couple of million years, you would likely wind up with a creature with wings, a lighter skeleton, and a relatively upright posture.

Try not to think of evolution as an enormous leap from A to Z, but rather a series of very little ones, with a bit of change occuring at each step.

I know we arent going to get a blow by blow record. but given that evolution is a slow process, wouldnt we have more examples? Has someone done some sort of analysis on what percentage of any given population we have fossils for? Based upon that, how many transitional fossils we should have? And whether our fossil sample is actually representative of what it should be given the current state of the theory?

See my section on percentage of land creatures vs marine ones, and why so few fossils are found. We can't calculate the exact number of transitions, of course, but from DNA of surviving animals (and perhaps those of creatures that were preserved), it is possible to calculate the degree of divergence (for example, it has been calculated that the genetic divergence between humans and chimps is a mere 0.05% - which means that chimps are closer to us than they are to gorillas. And no, there isn't one zero too many in that number!) and build a phylogenetic tree, and even estimate how long ago it was that the creatures diverged, based on a reliable molecular clock. (I won't go into details, but if you're interested, I could explain it in a later post) And then of course, based on this, you could fit in all the fossils you've found, with the aid of geochronology.

kfarlander wrote:

Well, had we still been using the carbon-14 dating method, I would have to agree. But since the 1950s there has been the development of three new clocks for dating: the uranium series, the rubidium/strontium and the potassium/argon, the last of which is by far the most accurate and is able to give reliable dates even for young rocks (of a million years of age or less), given that it has a half-life of 1,300 Ma, as opposed to the 5,730 Ma of the carbon-14 clock. Not to mention it can reliably date very small samples. (sample sizes, not size of samples)

I take it that this is your field.

Actually, no, but I did physics in high school, and I was always a big palaeontology nut.

kfarlander wrote:

Of course there are, you doint! What did you expect, a complete record of how the cranium size of this animal increased/decreased, millimeter by millimeter?

would be nice, but certainly not expected. What is reasonable to expect is that our records lie within boundries defined by statistics. Do they? You tell me given that this appears to be your area of expertise.

I don't claim to know the lineages of all flora and fauna, but to the best of my knowledge.... yes. If what you mean is that based on studies of the fossils they outline what might be found between A and B, and then some palaeontologist uncovers fossils that fits right in there. Palaeontology is like a jigsaw puzzle game where you're only given several pieces at a time.

Hipparion sounds like some sort of cloven hooved animal, but I am completely guessing

Yeah. It's actually one of the species that branched out from the same ancestor that gave rise to the modern horse.

kfarlander wrote:

At any rate, consider this: When you get right down to it, every single species is a transitional one! Think about it. What is evolution, if not a bushy series of transitionals?

yes, on some levels this is true. Even if you want to look at such small things(small time frame) like average human height.

Well, human height is probably not the best example, since it's a case of multifactorial inheritance. (Instead of the classical Mendelian two - T for tall and t for short - you have a fair number of them, so your height would depend on how many of which you get) But yeah.

kfarlander wrote:

BEEP - WRONG! The debate between scientists isn't whether evolution occurs (they all know it does), but whether the evolution is progressive/gradual, or whether it's punctuated (as in, a couple of graduals, and then a big leap, and then a couple more graduals etc).

That in and of itself is enough to perk my interest, given that I always believed it to be a slow gradual process.

Well, there were debates on this issue between the puncuationalists (like Stephen Jay Gould) and the gradualists (Richard Dawkins etc); I don't know if the thing is resolved. I doubt it!

Hope this helps.

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KittyKitty wrote:

Shotgun Assassin wrote:

Well, about evolution, one thing has got me scratching my head.....

How did humans evolve from having, what 1/2 chromosomes as the single celled organisms that started life, to having 46?

It seems very unlikely that, due to genetic mutation, a species that gave rise to mammals had 6 (or whatever) chromosomes, could mutate into a species with 8 chromosomes, AND breed with another one of its kind, which also has 8 chromosomes to start up that brance. Only for that species of 8 chromosomes to change again to a species with 10chromosomes.... etc.

Am I right in guessing this or is the answer something else?

Well, I know that certain things we would tend to call "birth defects" have to do with different patterns and numbers of chromosomes. I could swear that some of them actually wound up being a "surplus", or rather, the person in question actually has more than the standard 23 pairs. I might be mistaken here, since I'm by no means an expert on the subject, but I certainly seem to recall reading it somewhere.

If I'm remembering right, and we see this sort of thing even today, it's not hard to see how over the course of (millions of) years, we could have evolved that much.

If not.. well, I'll admit, it's a rather big change, but since again, I'm no expert, I'm certainly not going to say it's impossible - even if it may be a bit hard to completely understand.

Diinya-chan wrote:

In all seriousness, I was raised in a bible-believing household (and I serve YHWH too, if anyone cares... ) , so Creationism is my belief.

I have a six-hour lecture by a gent named Carl Baugh (sp?) on the subject, not to mention a crapload of Christian Apologetics lectures, if anybody wants to hear our side of the debate.

I don't know what YHWH is.. though I may easily be missing something terribly obvious lol... I used to be fairly religious.. During my middle-school and high school years, I (voluntarily) attended weekly youth group, went to a church of my own choosing, and did a lot of community service projects with others of my parish. Over the years though, I've really lost any interest in organized religion of any kind, though I still have my own faith... but it's pretty unorthodox by most standards.

Anyway.. all that is some background to explain my standpoint of not going in for the "well God said it, it must be true" idea (not that I'm saying everyone who has religion thinks that way, but it's been the majority in my own experience), but I'm always open for debate or discussion.

My personal opinion has been for quite some time that creationism was more than likely a parable or fable used to explain concepts that early civilization had trouble grasping, but that doesn't mean I'm unwilling to listen to anyone elses point of view on the subject. Though I'm not sure I could sit straight through a six hour lecture.. that takes some stamina lol

-Kitty

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Kent wrote:

Well crap. Here I came in expecting a discussion on the essence of science, the evidence for evolution etc, and then kfarlander already did all of the work...

Aw I'm sorry Kent. But hey, there's plenty more to discuss about evolution than I've put forth (which is just skimming the surface) - jump right in!

imurangel91 wrote: i dont believe in evolution.i think it is a load of crap .i dont like 2 believe that my ancestors were monkeys .how about u ?

Apes, not monkeys, imurangel. They're completely different animals! Monkeys and apes split up during the Oligocene epoch (about 38 million years ago). The African great ape and human lines diverged only about 5-7 million years ago during the Miocene epoch. And mind you, genetic studies have shown that our genome (ok ok DNA) is 99.5% similar to that of chimps'! If we diverged from them 7 million years ago, and still resemble them so closely in terms of DNA.... think of just how closely related we are to everything else that is alive.

Of course, I don't expect these numbers to fill you with a sense of wonder; perhaps when you're older and have studied biology...

Shotgun Assassin wrote:

How did humans evolve from having, what 1/2 chromosomes as the single celled organisms that started life, to having 46?

It seems very unlikely that, due to genetic mutation, a species that gave rise to mammals had 6 (or whatever) chromosomes, could mutate into a species with 8 chromosomes, AND breed with another one of its kind, which also has 8 chromosomes to start up that brance. Only for that species of 8 chromosomes to change again to a species with 10chromosomes.... etc.

KittyKitty wrote:

Well, I know that certain things we would tend to call "birth defects" have to do with different patterns and numbers of chromosomes. I could swear that some of them actually wound up being a "surplus", or rather, the person in question actually has more than the standard 23 pairs. I might be mistaken here, since I'm by no means an expert on the subject, but I certainly seem to recall reading it somewhere.

Well, acquisition of extra chromosomes is a fairly common occurrence in life. As Kitty has pointed out, there are cases whereby a human being acquires more than his fair share of 46 chromosomes. These cases are called 'trisomy' (as in one set having a triplet instead of a pair), and are caused by failure of the pair to separate during meiosis (cell division producing gametes - or sex cells). So far, all these cases have resulted in physical 'defects' in the progeny: for example, Down syndrome is caused by trisomy 21 (which is to say, the child has three copies of chromosome 21); Edward's syndrome is caused by trisomy 18. Interestingly, there was ONE documented case (see Asimov's Guide to Science) of a girl who possessed 48 chromosomes and lived.

Of course, in many cases large mutations like these are detrimental to the organism; however, once in a while you will get a mutation that is of benefit to the organism. Take for example, there is a blood defect illness called Sickle Cell Anemia whereby the victim's red blood cells are sickle-shaped as opposed to the normal round shape, and therefore are not as efficient at transporting oxygen. About 1 in 500 African Americans are affected. NOW here's the interesting bit: this genetic defect (which you'd have thought was good for nothing but causing people pain) actually confers upon its 'victims' resistance to malaria - those suffering from SCA in Africa were hardier and more likely to survive malarial outbreaks than their normal counterparts - which implies that this genetically abberarant blood cell evolved as protection against the disease. Once again, you're seeing evolution at work.

Perhaps it is difficult to comprehend how a one-chromosome organism could evolve to become a 48-chromosome one, but remember, this took billions of years. And billions of genetic mistakes, among other things. And in case anyone finds it difficult to imagine a one-chromosomed creature becoming a two-chromosomed creature.... Well, this was probably one of the first cases of symbiosis in the history of evolution. Imagine a single-celled predator eating a single-celled prey. Now, of course in most cases, the prey just becomes fodder - BUT! in a small percentage of these cases, the prey was able to resist digestion, and form an unlikely partnership with their hunters - the hunters providing the prey with shelter, and the prey processing the waste inside the hunters as food, or aiding their digestion. Eventually these partners evolved to the point where they could no longer live without each other; the internal partner, no longer needing its outer layer, lost it, and 'became one' with the larger cell. We carry living proof of this partnership in all our cells - our mitochondria, which are the energy plants of our cells, were once such 'prey' microorganisms. (George Lucas apparently filched this idea for Star Wars I; didn't anyone wonder why 'midi-chlorians' sounded so familiar? )

[As further evidence of this partnership: I'm sure that those of you who took biology will have learnt the difference between prokaryotic and eukaryotic cells. Prokaryotic cells are those lacking nuclei; their DNA bobs freely in the cell; bacteria are all prokaryotic cells. On the other hand, eukaryotic cells are those where the DNA is encapsulated in a nucleus; higher microorganisms like fungi, and multicellular organisms have eukaryotic cells. Fossil records show that nucleated cells just appeared out of nowhere, and were distinct from non-nucleated ones (ie. there were no intermediates); furthermore eukaryotic cells contain highly complex 'organelles' which were able to carry out internal self-reproduction independently of the cell; furthermore these strange little organelles had ribosomal subunits that were vastly different from the (eukaryotic) cell's, and highly resembled that of bacteria ribosomal subunits - both had subunits of size 30S and 50S, as opposed to our 40S and 60S --- all of which indicates that the event discussed in the above paragraph occurred.

And as to how multicellular organisms evolved: well, let's just say that it started out as a bacterial confederacy whereby the participant cells cooperated in a task... this led to an organisation of cells, a cell government, if you will, whereby these participants' organelles became integrated into a new biological unit. You can still see evidence of primitive coalitions like this still existing in nature. Slime moulds are, well, microbes that under normal circumstances antisocial creatures and live separated from each other. HOWEVER when conditions become unfavourable for it, you see an amazing phenomenon whereby all these slime mould cells come together to form a large fruiting body, to produce spores that they can disperse - so that the spores, which will germinate into slime mould cells can find new places to live and new food sources. This amazing phenomenon of nature is called Quorum Sensing, which is basically scientific jargon for 'microbe-speak'. Yes, microbes talk to one another, didn't you know? (I wrote an article for BBC's H2G2 entitled 'Small Talk in the Microbial World - you can find it HERE)

Yikes! Where has my brain wandered!

In any event, before I stray too far, I might also point out that polyploidy (having more than two pairs of chromosome sets) is a common occurrence in the plant world.

Forgive me for ranting out of control; I have sinned. Mea culpa!

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Shotgun Assassin wrote:

Its not hard to understand....

its just that the likelihood that two different members of the same species will BOTH have an extra pair of chromosomes at the same time,
THEN be healthy,
THEN be capable of breeding,
THEN meet
THEN breed,
THEN have thier offspring capable of breeding
THEN their offspring breed!

Its the culmination of a lot of highly unlikely coincidences.

hmm... unless....

Could evolution be speeded up by harsh circumstances? Perhaps some chemical causing extra pairs of chromosomes to pop up in localised areas?

You ask how likely it is that two members of a species (I assume you are also inferring that they are of opposite sex) gain extra sets of chromosomes, and still remain fertile. NOTE: polyploidy (more than two sets of chromosomes) is not to be confused with aneuploidy (more than two copies of a given chromosome). I'll discuss the two separately.

There are several mechanisms by which the number of chromosomes in an organism can increase or decrease:

Polyploidy

Polyploidy refers to the phenomenon whereby an organism possesses more than two sets of chromosomes. Plants are notorious for this . Interestingly, in the animal world, polyploidy is common in amphibians. Female wasps and bees are diploids; their male counterparts are usually haploid (having only one set). Different species of fish have varying degrees of polyploidy - up to hexaploid, for a certain type of carp! It is even possible in birds. Furthermore, it has been argued that polyploidy was the method of increasing the number of chromosomes in our invertebrate, fish and amphibian ancestors. It has been suggested that these extra sets of chromosomes, being surplus material, are then left to their own devices to mutate to produce different genetic sequences - which would lead to production of different proteins - which would lead to different phenotypes.

How do polyploid creatures breed? Well, there are those that do it the conventional way - and those that don't.

Not all organisms breed by sex as we know it (as in the mother and father each contribute one set of chromosomes to make two). There are quite a number of animals that reproduce by a mechanism called parthenogenesis, whereby the ovum develops into an individual without fertilisation by the sperm. Some fishes, insects and lizards reproduce by this method; some exclusively, and some only when the food supply is adequte (this is because this form of reproduction is faster than sexual reproduction, thus enabling the species to quickly exploit the available resources). A fraction of these need sperm from the male counterpart to stimulate egg development (although it does not contribute to the genetic composition of the progeny).

Parthenogenesis does not normally occur in mammals because of a phenomenon called 'gene imprinting' (which is the determination of the expression of the gene by the parent that contributed it; this violates the usual rule of inheritance - see my earlier chapter on genetics). HOWEVER, by circumventing gene imprinting, scientists have successfully induced some species of mammals to reproduce by this method, implying that there may have been some time in our past when mammals were capable of doing this on their own.

[[Why this gene imprinting, you ask. Because there are cases in which inheriting two functional copies of a certain gene is fatal or detrimental to the organism. For example, in humans, inheriting two working copies of the IGF2 gene - when only the father's copy should be expressed - will result in a cancer called Wilm's tumour. This mechanism may have evolved as a self-preservation method against the lethal gene effect. For example, the dominant gene dictating 'taillessness' in Manx cats is fatal to kittens if it is found in a pair; for that reason, all Manx cats are heterozygotic. Deactivating certain genes may be one of nature's ways of troubleshooting such genetic problems]]

Freaking hell, I've let my mind wander. Sorry! Let's go back to the bit about increase and decrease in numbers of chromosomes.

Chromosome Fission

This is another proposed mechanism of how the number of chromosomes can increase, whereby certain chromosomes split into two. This bit is a little beyond me, I confess; you'll have to go read it up on your own.

Chromosome Fusion/Translocation and Aneuploidy

In normal meiotic cell division, there is a phenomenon called chiasma, where pairing chromosomes perform genetic 'cross-overs' with their chromatids - which is why you won't get two individuals that are genetically the same (assuming sexual reproduction is involved). HOWEVER, there are cases whereby chromosomes that were never meant to pair up DO, and happily exchange segments like normal - with the result that you get two chromosomes that are very different in size. For example, if this were to happen between a chromosome with a long arm and short leg and a chromosome with a long leg and short arm, you would wind up with a huge chromosome with long arm and leg, and a dwarf with both short arm and leg. In extreme cases like these, the tiny chromosome might even be lost, resulting in one less chromosome. Oops.

Jokes aside, this is what most likely happened during the course of evolution of apes. When scientists compared human and ape chromosomes, they noticed that the end of the human chromosome number 2 (ie. the second largest one) very much resembled the long end of a pair of gorilla/chimp ones, implying that there was genetic loss on the side of the humans by this mechanism.

Now let's bring in aneuploidy. Meiosis refers to the process whereby gamete cells (sperm and ovum) are produced; unlike mitotic division of somatic cells, gamete cells will each only possess one set of chromosomes. On rare occasions, however, certain chromosomes may fail to separate during this cell division - which results in one gamete having an extra chromosome (it being a pair with the one it didn't part with), and the other gamete minus one chromosome. This is how 92% of Down Syndrome cases come about. [[Alternately, in a small number of cases, this trisomy is brought about by mistakes made during the early part of cell division of the developing zygote. (In which case the offspring will become a mosaic or chimaera)]]

You now have means of reducing the numbers of chromosomes. Now what?

Now! We have addressed the issue of difference in chromosome numbers not necessarily preventing breeding between species. I've already pointed out mules (donkeys x horses). In most cases, the offspring of such unlikely matches are sterile. Interestingly, however, hybrids of wild horses (33 pairs) and domesticated ones (32 pairs) produce offspring which, though possessing 32.5 pairs of chromosomes, are nevertheless fertile! How do animals that have undergone non-disjunctions or translocations manage to reproduce? In the case of these hybrid horses, the translocated segments of different chromosomes are still able to pair up and perform chiasma despite the fact that more than two centromeres may be involved; thus normal reproduction is possible for them.

But! What happens when you have nobody else to breed with, as Shotty said earlier? Well, it could be tough if you were the only one with x+1 chromosomes when everybody else has x. BUT we've solved the translocation problem; as for the aneuploidy one - well, it'll happen more than once. (For example, the incidence of Down syndrome in offspring increases dramatically with maternal age) So you've got a bunch of siblings with the same genetic abberation as you, with nobody else to mate with either. So what do you do? You interbreed.

Ghastly as this may sound (!), this is the most probable scenario in the early evolution of our human species. Comparative genetics studies have shown that our entire human race has about as much genetic diversity as a population of chimps. Oh dear.... as they say, incest is relative. Oh well.