Doubting Didymus

Welcome back scigirl. By the way, Do you know offhand where in the lineage the great apes lost their vitamin C synthesis? I had thought it was fairly recent, but if even orangutans have it I must be wrong. Do apes that divirged as far back as baboons have the broken gene?

Albion

I just googled on vitamin c and primates, and it seems to be saying that all primates (and guinea pigs and a few bird species) lack the ability to synthesise it. That includes at least one monkey species they've tested.

[ August 16, 2002: Message edited by: Albion ]</p>

Oolon Colluphid

The guinea pig one is via a different mutation I believe. I'll check for the reference. I hadn't heard about birds, but it's not too surprising. I wonder whether anyone's tested fruitbats...?

Oolon

Darwin's Finch

*Bump*

Mageth

I googled as well, and some bats also can't naturally produce Vitamin C (as well as at least some fish).

Vanderzyden

Sure, scigirl. By the "blushing" comment, I'm sure that you are far more knowledgable about these particulars. So, yes, I'd like to what you have to say regarding fusion. If convenient, you may simply direct me to your BEST concise reference on the matter at hand (other than the website already given). There's no need for a "general" biology lesson. We are talking specifics here, correct?

scigirl

Hello Vanderzyden,

Thank you for your reply. The reason I asked if you had any biological questions is that this evidence is particularly strong for evolution of humans, but in order to understand why, it is imperative that you have a basic understanding of genetics. If this post is confusing, please don’t hesitate to ask for clarification!

I’ll place this picture here again:



H = human (note there's only one), C = chimp, G = gorillla, O = orangutan. Note the three other primates have two chromosomes.

Basically, here’s the story. After Darwin formulated his theory of evolution, scientists went to work categorizing evolutionary trees. That is – they tried to figure out what evolved from what, based on morphology and the fossil record. Keep in mind that this was all done before we even knew what DNA or chromosomes were (although the theory of evolution did predict “heritable units.”) After the field of genetics developed, many scientists re-analyzed these trees with this new evidence. By and large, the trees still held true.

One of the trees that was re-analyzed speculated that humans and chimps shared a common ancestor. Now that we have it, does the chromosome data support or refute this tree? Let’s go over what evolutionary theory predicts:

Prediction 1. Humans and chimps should have the same number, or a very similar number, of chromosomes. Similar is of course a relative term, but the number of human chromosomes should match up better with a chimp, than say, a mouse, if evolution theory is correct. If the number is different, there should be a feasible natural explanation as to why they are different.

Well it was discovered that humans have 23 chromosomes in their gamete cells (sperm and egg cells that is). Chimps have 24. And there is evidence of a fusion event. If two chromosomes fused together, you would now have 23. Fusion events can be, and are, observed in nature, so this is a plausible explanation. So far, the tree still holds. I’ll explain more about evidence for a fusion later on.

Prediction 2. The chromosome patterns should look fairly similar to each other, when you compare patterns such as G banding and size.

Look at the picture above. G banding is a unique characteristic of each chromosome, much like a fingerprint. The G banding patterns of human and chimp chromosomes look similar or the same for the two sets of chromosomes. Chromosome 2 of humans looks like two of the chimp chromosomes put together.

You may argue here that, well, since genetics determines our morphology, than morphologically similar animals should have similar genetics patterns, right? That’s only true for some parts of our DNA, such as the genes. We would expect two similar-functioning proteins to have similar sequences. However, if evolution is not true, than it would be very surprising to find that the patterns of genes, and pseudogenes, and introns, and all that other genetics stuff is consistent with evolutionary trees.

Why would this be surprising? Because it appears that the patterns of genes themselves (which can be inferred by G banding) do not appear to be significant. In fact, genes seem to be all scattered around haphazardly if you analyze them according to their function. For instance, I used to work in a lab that studied an enzyme called “NADPH oxidase.” This important host defense enzyme is composed of five subunits. The genes for these subunits are all over the genome – which doesn’t make much sense if you buy into the “intelligent design” or “special creation” theory. However, if you buy into the theory that each subunit used to do something else, and became adapted over time for a new function but was still stuck in the same place in the genome (i.e. if you accept evolution) than this ‘haphazardness’ makes a lot more sense. Another piece of evidence that genes do not need to be in their exact spots in order to function is that you can transfect genes in on a plasmid into a cell or an animal, and by and large, they work ok.

So, if genes don't need to be in a particular order (as evidenced by genetic engineering AND just common sense observations about proteins), than why are so many genes from so many different organisms lined up in a pattern that is consistent with evolution?

What I am trying to impress upon you is this:

1) Genes do not appear to need a particular location in order to function.

2) Genes appear to have a ‘haphazard’ order if you analyze them according to what they do in a given organism.

3) These same exact genes are in that same weird haphazard order in related organisms. For example, the NADPH subunit gp91 is on the X chromosome in humans, and lo and behold, it's also on the X in all the animals that evolution says are our relatives.

If #1 is true, than why do we see #3, if not for evolution? If each animal were separately created, this to me seems like a pretty amazing coincidence that the gp91 subunit of NADPH oxidase is always on the X, even though we can show it doesn't need to be on the X.

Ok let’s get back to the chromosome data.

Prediction 3. If there was indeed a chromosome fusion event, there would be evidence of that fusion.

Every chromosome has two telomeres (one on each end) and a centromere in the middle. These telomeres and centromeres have characteristic DNA sequences to them. So, if indeed two of the chimp chromosomes fused in our ancestor, there should be evidence of 2 extra telomeres and 1 extra centromere in our chromosome # 2 (the one that matches the G banding patterns of two of the chimp chromosomes.

We indeed find just that evidence, as provided by nightshade in the original post. Here is the link again if you need it:

<a href="http://www.gate.net/~rwms/EvoEvidence.html" target="_blank">http://www.gate.net/~rwms/EvoEvidence.html</a>

As far as scientists can tell, the extra telomeres and centromere are not doing anything. They appear to simply be evidence of the fusion.
Many creationists have used the argument, “Well God made both humans and chimps so of course there would be similarities, just like a robot designer would make two similar robots.” In anticipation of that type of argument, let’s look at the robot analogy closer.

Suppose we are watching a junkyard wars marathon, and we tune in to find the following robot had been constructed:

1. A robot with a one-piece arm. This part had a hook on it that attached it to the main frame, and also had a characteristic blemish on it--say, a big red stain.

Now, let's say that the old creations get left in the junkyard, and can be used in future shows.
Next week we find this robot:

2. This robot body is much different from last week's robot. The arm is composed of two pieces this time. The piece that attaches to the main frame (the "upper arm) has a different hook from the first robot arm. But then we notice that there's a piece that was welded by the team to the upper arm (i.e. the "forearm) that, strangely enough, looks just like the main arm from the first robot. In fact, the hook is still there, but is not hooking to anything. Also, this forearm has the same red stain, and is the same size.

Would you conclude the following?

A. The junkyard wars team constructed the second robot arm de novo, they stuck a hook on the forearm even though it wasn't hooking to anything, and also purposely put a red stain on it.

B. The team found the first robot in the junkyard, took the arm, and stuck it on their new robot.

That, I believe is a better analogy. Robot A's arm represents one of the chimp chromosomes (how about 2q), robot B's arm represents human chromosome 2. A piece of chromosome 2 looks just like 2q - same size, same blemishes, same non-functioning hook. The centromere does actually function as a hook--the microtubules attach to it to pull chromosomes apart during meiosis and mitosis. Chromosomes only need one. We have not observed any known function for the extra centromere in the human chromosome, but yet, there it is, looking just like the centromere from 2q, blemishes and all.

Any questions?

scigirl

scigirl

Heh I know you asked for a "concise" reference on a very complex biological topic.

That's as concise as I get. My thesis was over 100 pages long!

scigirl

pz

That's all? And they still let you graduate?

Hmmph. This new generation just doesn't know how to hold up the proper standards of prolixity and pedantry.

Coragyps

Utterly OT, but I heard the story that De Broglie's thesis, on the wavelength of particles, had a title page and a page of text. The bastard!