Freethought & Rationalism Archive

Beer God · 05-12-2003, 07:23 PM

I asked a question, "Can An Evolutionist Explain this" and got some brilliant answers, but this is something I personally would like to know; if there is any resolution to this problem.

I don�t think there is an answer for this one. I really don�t mean to start quoting www.creationscience.com/ but they actually phrase this problem quite well.

Quote:

Handedness: Left and Right

Genetic material, DNA and RNA, is composed of nucleotides. In living things, nucleotides are always �right-handed.� (They were initially named �right-handed� because a beam of polarized light passing through them rotated like a right-handed screw.) Nucleotides rarely form outside life, but when they do, half are left-handed, and half are right-handed. If the first nucleotides formed by natural processes, they would have �mixed-handedness� and therefore could not evolve life�s genetic material. In fact, �mixed� genetic material cannot even copy itself.a

Each type of amino acid, when found in nonliving material or when synthesized in the laboratory, comes in two chemically equivalent forms. Half are right-handed, and half are left-handed�mirror images of each other. However, amino acids in life, including plants, animals, bacteria, molds, and even viruses, are essentially all left-handed.b No known natural process can isolate either the left-handed or right-handed variety. The mathematical probability that chance processes could produce merely one tiny protein molecule with only left-handed amino acids is virtually zero.c

A similar observation can be made for a special class of organic compounds called �sugars.� In living systems, sugars are all right-handed. Based on our present understanding, natural processes produce equal proportions of left-handed and right-handed sugars. Because sugars in living things are right-handed, random natural processes apparently did not produce life.

If any living thing took in (or ate) amino acids or sugars with the wrong handedness, the organism�s body could not process it. Such food would be useless, if not harmful. Because evolution favors slight variations that enhance survivability and produce more offspring, consider how advantageous a mutation might be that switched (or inverted) a plant�s handedness. �Inverted� (or wrong-handed) trees would proliferate rapidly, because they would no longer provide nourishment to bacteria, mold, or termites. �Inverted� forests would fill the continents. Other �inverted� plants and animals would also benefit and would overwhelm the balance of nature. Why do we not see such species with right-handed amino acids and left-handed sugars? Similarly, why are there not more poisonous plants? Why doesn�t any beneficial mutation permit its carriers to swamp most predators? Beneficial mutations are rarer than evolutionists believe.

source: http://www.creationscience.com/onlin...ciences40.html

(I feel I have dirtied Internet Infidels by quoting creationscience.com, sorry!)

Evolutionists and scientists in general have no consensus on this issue, some believed it simply occurred through random chance (which seems unlikely, though not impossible). Is this still one of the Achilles hells of evolutionary theory?

NZAmoeba · 05-12-2003, 11:06 PM

Quote:

why are there not more poisonous plants?

That ones easy to shoot down, a lot of plants rely on animals to spread seeds around, either by contact, or by eating and pooping it elsewhere.

Don't have time to read the rest sorry.

NZAmoeba · 05-12-2003, 11:33 PM

Quote:

Other �inverted� plants and animals would also benefit and would overwhelm the balance of nature.

Hence if they existed, they would have died. Overpopulation and limited resources can be factors in mass extinction, a typical example is bacteria in a dish, their population will increase exponentially, and when they hit peak, they'll die just as rapidly.

As for further scientific depth, i'll leave that to the more smarter people on this board...

Wounded King · 05-13-2003, 02:22 AM

Well at least they admit there are such things as beneficial mutations, that makes a nice change.

A pubmed search for evolution and homochirality should prodeuce a number of hits covering both amino acid and nucleotide handedness. Adding review to the search will cut out all the original reearch papers.

ps418 · 05-13-2003, 05:48 AM

Quote:

Genetic material, DNA and RNA, is composed of nucleotides. In living things, nucleotides are always �right-handed.� (They were initially named �right-handed� because a beam of polarized light passing through them rotated like a right-handed screw.) Nucleotides rarely form outside life, but when they do, half are left-handed, and half are right-handed.

I'm not sure about nucleotides, but there are definitely chiral-selective peptide systems, for instance, that could result in selective replication of one form starting from a heterochiral mixture. See New Study By Scripps Scientists Sheds Light On Key Step For Origin Of Life and How life got the upper hand.

Quote:

Synthetic organic materials, usually contain roughly equal numbers of the left- and right-handed versions of chiral molecules (known as L- and D-enantiomers). Life, on the other hand, is built almost exclusively from L-amino acids, and is said to be 'homochiral'.

Ghadiri's team used peptides -- the smaller cousins of proteins -- to investigate why homochirality dominates the biological world. The results showed that homochiral molecules readily self-replicate and so can rise to dominance in an initially heterochiral mixture.

The researchers used four different peptide fragments, each containing only D- or L-amino acids. Four different products were in principle possible: two homochiral, comprised of only D- or L-amino acids, and two heterochiral, made from two differently handed fragments.

But on mixing the fragments, the researchers obtained predominantly homochiral products. Once formed, the homochiral molecules acted as templates, accelerating their own replication, the team says. The heterochiral molecules, in contrast, did not use this ploy to compete.

Replacing even one of the about fifteen amino acids in, say, an all D-fragment by a single L-amino acid, suppressed the coupling reaction that would incorporate this 'mutated' fragment into a product. So the system churned out mainly homochiral products made of L -amino acids.

This suggests that life's homochirality could have arisen without the need for a large excess of L -- or D- molecules in the starting material, notes Lawrence Barron, a chemist at the University of Glasgow in the United Kingdom. "So theories involving the accumulation of chiral molecules through circularly polarized light, or magneto-optic effects, seem less relevant," he says.

Patrick

MrDarwin · 05-13-2003, 06:16 AM

Quote:

Similarly, why are there not more poisonous plants? Why doesn�t any beneficial mutation permit its carriers to swamp most predators? Beneficial mutations are rarer than evolutionists believe.

Poisonous to what or to whom? Humans? Mammals? All plant-eating animals?

The answer is quite simple: there are lots of poisonous plants. The problem is that what is poisonous to one animal is not poisonous to another. Think about chocolate and dogs, or monarch caterpillars and milkweed. Yew (Taxus spp.) foliage is quite toxic to humans and to most other mammals, but deer eat yews with relish.

Plants produce all kinds of chemical compounds that are not essential to their immediate survival, but most plants have to cope with "predation" by many different organisms, like various non-human mammals, insects, and arachnids (especially mites). There are even herbivorous reptiles and birds. And that's not even counting things we don't normally think of as "herbivores" like bacteria, rusts, and fungi. But because these different groups of organisms have different internal metabolic workings, what is poisonous to one group is frequently not poisonous to another. In the case of insects, many of which are specialists on particular plants or groups of plants, there are so many different classes that have evolved over such a long time that each one requires a different set of poisons. Insects and other invertebrates have such short generation times, they can often evolve to get around whatever toxins the plant is producing--it's the exact same principle as mosquitos and other insects evolving resistance to pesticides.

And poisons are metabolically costly to produce. A plant can produce one or two kinds of poisons, but it could never produce the full range of dozens or hundreds of kinds of poisons it would need to be poisonous to everything and everybody, nor would it need to. A plant doesn't need to take over the world, it only needs to survive long enough to produce a new generation.

Interestingly, the patterns of chemical compounds correlate extremely well with phylogenetic relationships, e.g., members of certain families of plants generally produce related groups of alkaloid compounds (and many of these correlations were discovered only after the phylogenetic relationships had been worked out, thus providing strong support to hypothesized relationships).

MortalWombat · 05-13-2003, 10:47 AM

Quote:

Originally posted by MrDarwin
Yew (Taxus spp.) foliage is quite toxic to humans and to most other mammals, but deer eat yews with relish.

I prefer mine plain. Or if anything, with just a dab of mustard.

caravelair · 05-13-2003, 01:10 PM

"Once the preference for one enantiomer over another gets started in nature, it is relatively easy to see how this preference is perpetuated. Biological reactions work much like machines having templates, stamping out the preferred, and ONLY the preferred configuration generation after generation after generation.

As to how one became initially started, there are many possibilities:

Luck. The first one to form just happened to be L, and then the rest followed.
There may be some effect during formation due to coriolis force or the (hemisphere dependent) magnetic field (as lightening went DOWN, the effect may be polarized)
Quantitative calculations indicate that the fundamentally left-handed neutral-weak force with the electromagnetic force could introduce an energy preference (very slight). Aside from any steric preferences, one form could be energetically more stable than the other.
[William C. McHarris Professor of Chemistry and of Physics and of Astronomy at Michigan State University "Handedness in Nature" January 1986 Analog]"

taken from here.

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05-13-2003, 02:22 AM	#4
Wounded King Veteran Member Join Date: Mar 2003 Location: Edinburgh Posts: 1,211	Well at least they admit there are such things as beneficial mutations, that makes a nice change. A pubmed search for evolution and homochirality should prodeuce a number of hits covering both amino acid and nucleotide handedness. Adding review to the search will cut out all the original reearch papers.

05-13-2003, 01:10 PM	#8
caravelair Regular Member Join Date: Apr 2003 Location: toronto Posts: 420	"Once the preference for one enantiomer over another gets started in nature, it is relatively easy to see how this preference is perpetuated. Biological reactions work much like machines having templates, stamping out the preferred, and ONLY the preferred configuration generation after generation after generation. As to how one became initially started, there are many possibilities: Luck. The first one to form just happened to be L, and then the rest followed. There may be some effect during formation due to coriolis force or the (hemisphere dependent) magnetic field (as lightening went DOWN, the effect may be polarized) Quantitative calculations indicate that the fundamentally left-handed neutral-weak force with the electromagnetic force could introduce an energy preference (very slight). Aside from any steric preferences, one form could be energetically more stable than the other. [William C. McHarris Professor of Chemistry and of Physics and of Astronomy at Michigan State University "Handedness in Nature" January 1986 Analog]" taken from here.

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