tea_and_biology
tea_and_biology t1_iuljyvz wrote
Well, first off, the big bang wasn't an explosion. The rapid expansion of space-time and condensed material and a legit boom-boom explosion are rather different things. Further, the (much) higher energy density in the moments immediately after the Big Bang hindered the process of element formation, rather than helping it.
At over a billion or so degrees kelvin, both protons and neutrons are too energetic to bind. Only once things had begun to expand and cool below this threshold, after about ~2 minutes, could they fuse, resulting in hydrogen (^(1)H) nucleosynthesis, and subsequent fusion into deuterium (^(2)H) and helium-4 (^(4)He). But if it gets too cool, fusion and nucleosynthesis stops altogether - so really, there was a critical period between approximately ~3 and ~20 minutes just after the Big Bang for all original element synthesis to happen.
After this short window, everything has cooled off, and you have a universe where ~25% of the mass is ^(4)He, and the other ~75% mostly ^(1)H - with teeny weeny dribs and drabs of ^(2)H, ^(3)He and lithium (^(7)Li) here and there. Given this composition, the only reactions that could form any heavier elements therefore include:
>^(1)H + ^(4)He → ...
>^(4)He + ^(4)He → ...
... but neither produces stable nuclei. There's only:
>^(2)H + ^(7)Li → ^(9)Be
>^(4)He + ^(7)Li → ^(11)B
But given lithium was so scare, these reactions were incredibly unlikely. Trying to build any heavier elements now becomes essentially impossible - the universe is too cool, and the stuff that's in it isn't super useful.
We needed to wait a helluva' long time for the first stars to begin forming things up to carbon (via the triple alpha process, over tens of thousands of years: ^(4)He + ^(4)He → ^(8)Be + ^(4)He → ^(12)C), subsequent fusion to get up to iron, and then supernovae for everything else. The secret ingredient here was time - stars can afford to wait and build up their cupboard of ingredients to get the fun recipes going, something the primordial universe lacked.
In short: During the Big Bang, it was too hot and dense for anything to form beyond the simplest gases, and then quickly became too cool for anything else to appear. Stars, by contrast, have plenty of time on their hands.
References & Further Reading:
tea_and_biology t1_iuoayu4 wrote
Reply to What is the oldest "surviving" strand of DNA? Could it be from the first living organism? by DumbNBANephew
> Would this original still exist today? Or would mutations have gotten rid of the original strand/chromosome a while ago?
Not a chance.
But what is the oldest 'surviving' hunk o' DNA still floating about? Well, let's interpret that in three ways:
i) Oldest DNA in a living organism:
As far as the actual nucleotide molecules within a given genomic position are concerned, the turnover rate is incredibly high - at least half is 'new' for that double-stranded sequence with every division cycle, on the order of at most, usually, days for most metabolically active tissues in most organisms. Coupled with DNA damage repair, nucleotide salvage, and other mechanisms recycling and replacing individual nucleotides as and when, this is pure conjecture* but I'd say it's almost a statistical certainty that no given single nucleotide will be passed on inter-generationally and remain exactly where it was in sequence.
This means that, with the most conservative estimates, I'd imagine the 'oldest' stretch of DNA on Earth that has remained atomically conserved in a living beastie is going to be something like a teeny stretch of several adenosines tucked away in a quiescent cell in some long-lived deep sea clam, or something. A few decades, maybe one or two hundred years at most?
Who knows, to be honest. Not very long in any case!
EDIT: Oh, wait. I totally forgot about plants. In which case, maybe swap clam for some old gnarled bristlecone somewhere. Saying that, IIRC there's proportionally more live tissue turnover in modular organisms like plants compared to unitary animals (i.e. most of the substance of a tree is dead xylem and excreted lignin; the 'alive' bit is the relatively thin film of phloem in the bark over the surface, constantly replicating outwards). I really don't know. Still same-ish order of magnitudes in years in any case.
^(^* I tried doing a quick sweep of the literature to find decent replacement rates; thought the radioisotope labelling stuff would be fruitful, but alas, nothing. If anyone has a source for molecular (not sequence)^) ^(replacement rates, we'd be able to narrow this down to an informed range, instead of pure speculation.)
ii) Oldest DNA on Earth:
It's different for DNA stuck in dead things though. Without biological processes to generate nucleotide turnover, things are more promising, though we're still at the mercy of the chemical half-life of DNA which, condition dependent, has been calculated to be approximately ~512 years (i.e. every 512 years, the bonds in half the DNA in a given sample are broken down). This sets a maximum upper bound in the low millions of years (about ~6ish for 1% DNA to remain); indeed, the oldest fragment of ancient DNA we've sequenced to date is from a mammoth molar, approximated at being ~1.6 million years old (though including error up to a lowest confidence score at most ~2.25ish). Anything much older will likely have degraded to useless unsequence-able mush.
So yup, there's probably a bit of post-DNA nucleotide residue in some mummified subfossil somewhere that'll be a few million years old.
ii) Oldest conserved DNA sequence:
If we twist the question a little and forget about the actual physical DNA, but rather what it encodes, then we're really upping the numbers here. Many genes are super conserved across all life, as they perform functions which are foundational for all other biological processes (e.g. protein or ATP production). Assuming the most conserved are the oldest, candidates for 'oldest sequence' are the RNA sequences for the 16S and 23S ribosomal subcomponents, plus assorted tRNA sequences (all involved in converting RNA into protein), and then a bunch of genomic DNA genes for ATP-binding cassette (ABC) transporters, which are crucial for moving things in and out of membranes. The most conserved of these is within yecC, the human homolog being the TAP2 gene. In which case, the oldest bit of DNA sequence in your body right now, having been bobbing about in the last universal common ancestor of all extant life (LUCA), at least ~3.9 billion years ago, is this:
> AAAGGAGTCCAAATGTCTGGAGGACAGAAGCAAAGAAATTGCAATTGCTCGAGCTTTGATCCGTGATCCACGTGTTCTGATCATATACGACGAGCCAACTTCCGGACTCGAC
Ooooh. Check out them sexy Cs.
The originals are long gone, but the copies survive!
References:
Allentoft, M.E., Collins, M., Harker, D. et al. (2012) The half-life of DNA in bone: measuring decay kinetics in 158 dated fossils. Proceedings of the Royal Society B: Biological Sciences. 279 (1748)
Betts, H.C., Puttick, M.N., Clark, J.W., Williams, T.A., Donoghue, P.C.J. & Pisani, D. (2018) Integrated genomic and fossil evidence illuminates life’s early evolution and eukaryote origins. Nature Ecology & Evolution. 2 (10), 1556-1562
Isenbarger, T.A., Carr, C.E., Johnson, S.S., Finney, M., Church, G.M., Gilbert, W., Zuber, M.T. & Ruvkun, G. (2008) The most conserved genome segments for life detection on Earth and other planets. Origins of Life and Evolution of Biospheres. 38 (6), 517-533
Lane, A.N. & Fan, T.W.M. (2015) Regulation of mammalian nucleotide metabolism and biosynthesis. Nucleic Acid Research. 43 (4), 2466-2485
Valk, van der T., Pecnerova, P., Diez-del-Molino, D.D. et al. (2021) Million-year old DNA sheds light on the genomic history of mammoths. Nature. 591, 265-269