Meteor markers on our DNA?

[Loren Booda]

According to one estimate, our planet exposes itself to meteor showers in particular once every 26,000,000 years. Our genetic ancestors often had to adapt to the extreme conditions presented by such catastrophic potential. This included natural selection that could have passed down a special sequence on our genes.

Might segments of the human genome still carry the ability to survive apocalyptic epochs, and even favor some individuals specifically during a mass destruction?

 

[misskitty]

It would make sense this would be encoded incase we had to use it. I don't know about the mass destruction part though.

~Kitty

 

[Moonbear]

Genes aren't "proactive." They don't predict disasters. Rather, if there were a disaster, the survivors live on, and the rest are killed. That's natural selection.

 

[Ouabache]

I can believe a natural disaster such as, multiple impacts by large meteors (or asteroids) the subsequent global impact on our atmosphere and climate, that some organisms will survive but probably not humans. Not to worry though, those few survivors have a high likelihood to evolve into an equally intelligent being as ourselves.

 

[selfAdjoint]

Genes aren't "proactive." They don't predict disasters. Rather, if there were a disaster, the survivors live on, and the rest are killed. That's natural selection.

And overall, the survivors are those better fitted for surviving under those conditions. Their genes helped them to be so, and those genes were passed down to their descendents. Hence Loren's question. We are descended from animals which survived not one, but many such disasters, it would be amazing if we did NOT have something in our genome from that.

Did you know that it has been found that some of the people who are immune to HIV have genes that militate against a chemical stratagem the virus employs, and they have those genes because they are descended from survivors of the medieval black death plague, the bacillus of which uses the same stratagem.

 

[Ouabache]

Though the idea of maintaining and transmitting gene sequences that allow humans to survive future incidence of disease; as in the case with the http://www.pbs.org/wnet/secrets/case_plague/clues.html [Broken]. Assaults such as; tidal waves, global dust clouds, global famine, extreme cold and darkness. Well the last time one of those events happened, amoung the survivors were a few tiny rodent-like animals. So if we maintain sufficient plant and animal diversity on earth, the fittest will certainly survive.

 

[selfAdjoint]

Well the last time one of those events happened, amoung the survivors were a few tiny rodent-like animals.

Such is the common wisdom, but see the currrent Science issue for a tale of larger mammals living farther back in the Dinosaur era. And don't forget that the ancestors of the birds also survived the K-T disaster. Indeed I don't believe the pattern of which species survived and which didn't has ever really been explained. Maybe only large cold blooded animals? On land anyway.

 

[Phobos]

Not to worry though, those few survivors have a high likelihood to evolve into an equally intelligent being as ourselves.

nitpick...I wouldn't call it a high likelihood. There is no preset evolutionary path toward high intelligence. Evolution went on for something like 3.8 billion years before producing human intelligence (granted, that is starting from square 1), and we're a rare/unique branch in the evolutionary tree.

 

[Monique]

You are probably talking about impacts that would throw the planet in a blanket of darkness? Let's talk about such a situation specifically.

First look at a different situation. A pandemic happening in 1347-1350, killing up to a third of Europe's population: black death (plague). How would you estimate the effect that this had on our genome and what do you think has happened to the effect over the years that there hasn't been selective pressure.

The event was a true bottleneck for the population of Europe. Many people died, which led to a decrease of the gene pool. Survivors could repopulate all the different regions.
In this case there was a very specific biological threat that allowed the selection of individuals that were resistant. Today you could probably still identify the specific sequence that induced the resistance, but since there has not been continuous selection pressure, the resistance would be diluted with every generation.

Now back to your example. First you need to identify what biological characteristics would allow survival of a catastrophic meteor impact, there probably are a whole lot of things that would lead to survival. Do you think those things are meteor-impact specific? Why would you assume that there is enough selective pressure to maintain those biological characteristics over 26,000,000 years?

 

[Nereid]

Expanding on Monique's question - if the http://en.wikipedia.org/wiki/Nemesis_(star)" [Broken]* turns out to have no basis, does that in any way change the OP's question?

That there have been many, many catastrophies in the time of multicellular life on Earth is quite well established. That at least one mass extinction was due to an asteroid/comet collision is also well established (the KT).

Perhaps a good place to look would be the http://palaeo.gly.bris.ac.uk/Palaeofiles/Permian/intro.html" [Broken] - it made the KT one look like just a cloudy day.

*I don't endorse this Wiki article, it was just the first I could find that seemed to cover the bases sufficiently to introduce the idea. In fact, I think you'll find that, the Wiki article notwithstanding, the Nemesis hypothesis of Raup et al. is pretty much dead - no extinction periodicity in the fossil record, no periodic traces of meteor storms (or whatever), no sign of any such Nemesis star, ...

 

[Andre]

The correct cycle is 62 million years, not 26. according to that same Richard Muller here and this time the statistical significance is quite convincing. So probably no Nemesis, although this father here can't bury his child yet but lots of speculation for the cause of the 62 Ma cycle.

 

[Loren Booda]

Even if the cycle of assumed meteor strikes is 62 million years, effects such as blocked photosynthesis, flooding/dessication, dust-impaired respiration and extreme climate change could have been repeated dozens of times during the process of evolution. The effects themselves would be replicated over and over, so the needed adaptations in turn would conform with this repetition.

I agree that such a catastrophe might be less genetically specific than a pandemic, and occur much more infrequently. However, life has survived many of these extraterrestrial events, testing the fittest to their limit. Meteor impacts tend to eliminate more fundamental aspects of biota than the plague, perhaps equaling it overall in destruction of evolutionary development, and requiring extraordinary reparation.

 

[Ouabache]

Such is the common wisdom, but see the currrent Science issue for a tale of larger mammals living farther back in the Dinosaur era.

That sounds interesting.. Do you happen to know a link to that article?

nitpick...I wouldn't call it a high likelihood... we're a rare/unique branch in the evolutionary tree.

repickin' nits... Agreed, a high degree of intelligence does appear to be rare in our universe (based on current observations). By high likelihood, I mean given similar conditions (i.e. presence of life and time), intelligent life will eventually occur on earth. It may take longer or shorter period, but I would be pretentious to assume otherwise.

 

[Rade]

One way such impact could be recorded in mammal DNA is if the meteor(s) in question contained high levels of an isotope of an element that is rare on the earth, yet required for formation of DNA (say C, H, etc.--many possibilities). Here for example is a table of some stable isotopes common to living species: (from this site: (http://www.uga.edu/~sisbl/stable.html)

Table 1.1. Average Terrestrial Abundances of the Stable
Isotopes of Major Elements of Interest in Ecological Studies

Element Isotope Abundance (%)

Hydrogen 1H 99.985(%) 2H 0.015(%)
Carbon 12C 98.89(%) 13C 1.11(%)
Nitrogen 14N 99.63(%) 15N 0.37(%)
Oxygen 16O 99.759(%) 77O 0.03(%) 18O 0.204(%)
Sulfur 32S 95.00(%) 33S 0.76(%) 34S 4.22(%)
36S 0.014(%)

But how to sample isotope levels from DNA pre-post impact ? Perhaps better to look for isotopes of Ca in fossil bones of mammals pre-post impact time period ? Here are the stable Ca isotopes from internet site:

Most stable isotopes

40Ca 96.941% Ca is stable with 20 neutrons
41Ca {syn.} 103,000 y ε 0.421 41K
42Ca 0.647% Ca is stable with 22 neutrons
43Ca 0.135% Ca is stable with 23 neutrons
44Ca 2.086% Ca is stable with 24 neutrons
46Ca 0.004% Ca is stable with 26 neutrons
48Ca 0.187% >6×1018 y β- 4.272 48Ti

OK, now time to cut holes in the idea.

 

[Ouabache]

Intriguing thought, but you're right there are some gaping holes.

hole #1, There is no Ca in DNA.

hole #2, The information that transcends generations, is the DNA sequence, not the elements that comprise it. During replication, a new strand of DNA is made using chemical constituents presently available (whether recycled from within the body, coming from food or the air that is breathed). So if there were some rare isotopes initially present, they are soon diluted out after several generations, become homogeneous with the background.

 

[Loren Booda]

Relatively rare and short-lived radioactive isotopes, brought onto Earth in sufficient quantities and at regular intervals, may have imposed a cyclic stressor to adapt to.

 

[Rade]

Intriguing thought, but you're right there are some gaping holes. hole #1, There is no Ca in DNA.

Perhaps I can fill this hole--since it is known that some cations are associated with DNA--so if not Ca, perhaps Mg, which has three stable isotopes, see here:

Abstract
Annual Review of Biophysics and Biomolecular Structure
Vol. 32: 27-45 (Volume publication date June 2003)
(doi:10.1146/annurev.biophys.32.110601.141726)

First published online as a Review in Advance on February 14, 2003

CATIONS AS HYDROGEN BOND DONORS: A View of Electrostatic Interactions in DNA

Juan A. Subirana1 and *Montserrat Soler-López2*
department d'Enginyeria Química, Universitat Politècnica de Catalunya, Barcelona, Spain; 08028 email: juan.a.subirana@upc.es

2European Molecular Biology Laboratory, Grenoble Outstation, Grenoble Cedex 9, France; 38042 email: soler@embl-grenoble.fr

Cations are bound to nucleic acids in a solvated state. High-resolution X-ray diffraction studies of oligonucleotides provide a detailed view of Mg2+, and occasionally other ions bound to DNA. In a survey of several such structures, certain general observations emerge. First, cations bind preferentially to the guanine base in the major groove or to phosphate group oxygen atoms. Second, cations interact with DNA most frequently via water molecules in their primary solvation shell, direct ion-DNA contacts being only rarely observed. Thus, the solvated ions should be viewed as hydrogen bond donors in addition to point charges. Finally, ion interaction sites are readily exchangeable: The same site may be occupied by any ion, including spermine, as well as by a water molecule.

hole #2, during replication, a new strand of DNA is made using chemical constituents presently available (whether recycled from within the body, coming from food or the air that is breathed). So if there were some rare isotopes initially present, they are soon diluted out and homogeneous with the background, after N generations.

OK, a hole not so easy to fill--but consider possibility that the rare isotope from meteor was radioactive, with very long half-life, with constant decay over time to one of the stable cations discussed above that is associated with DNA. In that way, over many millions of N generations the isotope would be present in the replicated DNA sequence.