Searching under the lamp-post
By RICHARD DAWKINS - RICHARDDAWKINS.NET
Added: Mon, 26 Dec 2011 19:06:05 UTC - An RDFRS Original
See below for Polish translation
The joke is familiar. Man searches diligently under lamp-post at night. Explains to passer-by that he has lost his keys. “Did you lose them under the lamp-post?” “No.” “Then why are you looking under the lamp-post?” “Because there’s no light anywhere else.”
The argument has a certain zany logic, and it seems to appeal to Paul Davies, distinguished British physicist now at Arizona State University. Davies is interested (as am I) in whether our kind of life is unique in the universe. The DNA code, the machine code of life, is all but identical in every living creature that has ever been examined. It is highly unlikely that the same 64-triplet code would coincidentally evolve more than once independently, and this is the main evidence that we are all cousins, sharing a single common ancestor, which probably lived between three and four billion years ago. If life originated more than once on this planet, only one life form survives: our kind of life, typified by our DNA code.
If there is life on other planets, it will very likely have something equivalent to a genetic code, but it is highly unlikely to be the same as ours. If we discover life, say on Mars, the acid test of whether it originated independently will be its genetic code. If it has DNA and the same 64-triplet DNA code, we shall conclude that it is a cross-contamination, perhaps via a meteorite.
We know that meteorites do occasionally travel between Earth and Mars – and, by the way, here is my second example of searching under the lamp-post. A meteorite can land anywhere on Earth, but we are unlikely to find it lying on any surface other than permanent snow: anywhere else it would just look like a stone, and it would soon be covered by vegetation or dust storms or soil movements. This is why scientists hunting for meteorites travel to the Antarctic: not because they are more likely to be there than anywhere else, but because that is where you can clearly see them even when they landed a long time ago. Antarctica is where the lamp-post is. Any stone or small rock lying on top of the snow must have dropped there – and it is quite likely to be a meteorite. Some meteorites found in Antarctica have been shown to come from Mars. This astonishing conclusion follows from a careful matching up of the chemical composition of these rocks with samples taken by robot spacecraft sent to Mars. Some time in the distant past, a large meteorite hit Mars with catastrophic impact. Fragments of Martian rock exploded up into space and some of them eventually ended up here. This shows that matter does sometimes travel between the two planets, and this opens up the possibility of cross-contamination by (presumably bacterial) life. If Earth-life did contaminate Mars (or vice versa), we would recognise it by its DNA code: it would be the same as ours.
Conversely, if f we found a life form with a very different genetic code – not DNA, or DNA with a different code – we would call it truly alien. Paul Davies suggests that maybe we don’t need to go even as far as Mars to find truly alien life. Space travel is expensive and difficult. Maybe we should be searching right here for alien life that started on Earth, independently of ours, and never left. Maybe we should be systematically examining the genetic code of every micro-organism we can lay our hands on. Every one so far examined has the same genetic code as we do. But we have never systematically searched to see if we can find a different genetic code. Earth is Paul Davies’ lamp-post because it is much cheaper and easier to search among Earthly bacteria than to travel to Mars, let alone to other star systems where the best hope of alien life reposes. I wish Paul good luck in his search under that particular lamp-post, but I am very doubtful of success, partly for the reason Charles Darwin himself gave: any other life form would probably have long ago been eaten by our kind – probably bacteria, we can today add.
I was reminded of all this by a news story in today’s Guardian. ‘Scientists to scour 1m lunar images for signs of alien life.’ Yet again, the story concerns our old friend Paul Davies, and he is yet again down on his hands and knees, under yet another lamp-post.
If technologically advanced aliens ever visited us, they would be much more likely to have done so in the past than in the present, simply because the past is so much bigger than the present – if we define the present as one lifetime, or even as the span of recorded history. Traces of alien visitations – wrecked spacecraft, rubbish, evidence of mining activity, maybe even an intentionally deposited signal as in ‘2001: A Space Odyssey’– would quickly (by the standards of geological time) be covered over on the actively heaving and vegetation-covered surface of Earth. But the moon is another matter. No plants, no wind, no tectonic movements: Neil Armstrong walked in the lunar dust 42 years ago, and his footprints probably still look fresh. So, Paul Davies and his colleague Robert Wagner reason, it makes sense to examine every high resolution photograph ever taken of the moon’s surface, just in case traces are to be seen. The probability is low, but the pay-off could be very high, so it is worth doing.
I am very sceptical. I suspect that there is life elsewhere in the universe, but it is probably extremely rare and isolated on far-flung islands of life, like a celestial Polynesia. Visitations to one island by another are hugely more likely to be in the form of radio transmissions than visitations by corporeal beings. This is because radio waves travel at the speed of light, whereas solid bodies travel only at the speed of – well, solid bodies. Moreover, radio waves travel outwards in an ever-expanding sphere, whereas bodies travel in only one direction at a time. This is why SETI (Search for Extra-Terrestrial Intelligence using radio telescopes) is worthwhile. SETI is not wildly expensive as big science goes, but Paul Davies’ latest lamp-post is a lot cheaper and I again wish him luck.
Szukanie pod latarnią
Autor tekstu: Richard Dawkins
Tłumaczenie: Małgorzata Koraszewska
To znany dowcip. Człowiek szuka czegoś uważnie wieczorem pod latarnią. Wyjaśnia przechodniowi, że zgubił klucze. „Czy zgubiłeś je pod latarnią?". „Nie". „To dlaczego szukasz pod latarnią?". „Bo nigdzie indziej nie ma światła".
Argument ma pewien rodzaj uciesznej logiki i wydaje się odpowiadać Paulowi Daviesowi, wybitnemu fizykowi brytyjskiemu, pracującemu obecnie na Arizona State University. Daviesa (jak i mnie) interesuje, czy nasz rodzaj życia jest wyjątkowy we wszechświecie. Kod DNA, kod życia, jest właściwie identyczny w każdym żyjącym stworzeniu, jakie kiedykolwiek zbadano. Jest wysoce nieprawdopodobne, żeby ten sam kod 64 trójek wyewoluował przypadkowo i niezależnie więcej niż jeden raz. Jest to główny dowód, że wszyscy jesteśmy kuzynami, mamy jednego wspólnego przodka, który prawdopodobnie żył między 3 a 4 miliardy lat temu. Jeśli życie powstało więcej niż jeden raz na tej planecie, tylko jedna jego postać przetrwała: nasz rodzaj życia, czego symbolem jest nasz kod DNA.
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