Comment by credit_guy | Hacker News Reader

Comment by credit_guy | Hacker News Reader

Oh, man, what non-sense.

1. A DNA molecule is billions of pairs long. What does half-life measure? When does a DNA molecule count as destroyed? When one pair breaks, when 10% of the pairs break, when 99.9% of them break?

2. Even if pairs break, so what? When we sequence DNA from various organisms, we start by breaking the strands in small pieces anyway.

3. Even if this half-life had any meaning, it would not be a constant number. Half-life is a constant number for radioactive decay. For other things, it's just a somewhat helpful concept. How long do onions keep until they spoil? Maybe they have a half-life of one week. But, the storage conditions make a huge difference. If you put a lot of onions in a plastic bag, and leave them in a hot, humid and unventilated place, they'll spoil much faster than if you keep them well separated, in a dry and cool place. The same with DNA. In fact DNA's half life is probably days, not hundreds of years. In exceptional conditions, some animal or human remains get exposed to some conditions that make them "fossilize". So, if the 521 year half-life has any meaning, it's a conditional expectation given some exceptional situations. But then some situations are more exceptional than others. The 521 years (ha, just think of it, not 500, but 521! ) is already millions of times longer than the normal decay time of animal tissues. What is to say there are no more exceptional conditions under which the DNA half-life won't be 5 million years?

4. And even if 521 was an exact number, for every type of animal tissue, for all places in the world, that still does not mean we can't reconstitute the DNA's of the dinosaurs. Dinosaurs had both ancestors and successors. The successors are the birds, the ancestors lead to crocodiles, and maybe other animals as well. They have common ancestors with lots of other animals, or more precisely, with all animals. DNA is information. Information propagates. Science comes up all the times with better and better methods to retrieve information. Just look at how your iPhone is able to take a picture at night now, versus 10 years ago. Sure, part of that quality leap is the sensor, but a huge part is the Machine Learning algorithms used for denoising. In other words, for information retrieval.

1. From the article:

> By comparing the specimens' ages and degrees of DNA degradation, the researchers calculated that DNA has a half-life of 521 years. That means that after 521 years, half of the bonds between nucleotides in the backbone of a sample would have broken; after another 521 years half of the remaining bonds would have gone; and so on.

Oh, man, what negativity.

Even if the term “half life” isn’t really appropriate here, it’s clear enough (and explained well enough) in the article. This isn’t a published scientific paper, this is an article likely meant for a more general audience, so, jeez, cut it some slack.

Fair enough, I was too negative. I'll leave my comment unedited, so people won't get confused about the exchange.

The article itself is ok, and the researchers had good intentions. The problem is the idea took a life of its own. It almost became a meme. I heard this article many times quoted on HN. And the idea itself is definitely wrong, and needs to be debunked.

If the 521-year half-life were remotely true, we couldn't hope to ever sequence something older than 100 or 1000 half lives (i.e. half a million years). Well, we did that just the year after (this article was published in 2012, we sequenced a 500k year old fossil in 2013) We also sequenced a one million year fossil in 2021 [1])

[1] https://www.nature.com/articles/d41586-021-00436-x

You should read the article and not comment based on the title and your preconceptions. All of your questions are addressed in the text of the article.

Agree. It's unclear from the article that information cannot still be reliably obtained from the sample. Sure the bonds break, but since the construction of nucleotide bases are well understood, does that mean these cannot be distinguished in the sample?

Read the page. Half life is precisely defined.

Your claim half-life doesn't apply is incorrect. All atoms undergo radioactive decay, so half life applies as precisely to all matter as is does to uranium.

> All atoms undergo radioactive decay

Are you sure about that?

i guess he has never heard of a stable isotope.

Stable isotopes decay but at much slower rates. Even protons decay.

While neutrons do decay in the standard model (half life on the order of 10 minutes, observed), protons do not decay in the standard model, nor has proton decay ever been observed -- and folks have looked.

So far the isotopes called "stable isotopes" are ones with unobserved decay, and slowly over time ones that used to be called stable are being found to decay (e.g., Bismuth 209). Quantum uncertainty (and still theoretical proton decay, expected to happen, just not measured well yet) are expected to get the rest.

Ultimately quantum tunneling will get them all. There is no known mechanism to prevent it.

I think the term "effectively stable" is what is meant in this context.

Even if it were true that all atoms had a half-life, "DNA" is not an atom.

Doesn't matter. Half life is used for anything that is modeled with exponential decay: energy systems, medicine effectiveness, caffeine in a body, oscillations.

So if the essence of a thing breaks down, and the model for it is exponential decay, then it has a half life.

Thus DNA has a half life, just as the paper in Nature, one of the premier scientific publications in the world, demonstrates.

Nonetheless, "atomic radioactive decay exists and DNA is made of atoms" is not the reason DNA has its half-life, as your post claimed.

No where did I claim the decay is the one used in the article, in fact, my first sentence is "Read the page. Half life is precisely defined." I was well aware of their definition.

My explanation was to point out that even if you don't like the definition in the article, that there are other valid ways to define half-life for atoms and complex molecules.

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