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Are there any genes (for any organism) for which we can say with confidence that they are over a billion years old?

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Ribosomal RNA (rRNA) genes are shared among all living organisms, including lineages that have diverged from one-another well before a billion years. Bacteria, Archaea, and Eukaryotes diverged over 3 billion years ago, and their rRNAs are virtually identical to one-another. http://www.biologyreference.com/Ar-Bi/Archaea.html Any genes that are shared between plants, animals and fungi would also be over a billion years old.

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  • $\begingroup$ "…their rRNAs are virtually identical to one-another." No. Just counting the bases, typical examples are ssRNA: bacteria 1542 nt, eukaryotes 1869 nt; lsRNA: bacteria 2906 nt, eukaryotes 5070 nt. They are certainly related, but a nuanced explanation of how is required. See e.g. R. Brimacombe (1984) TIBS 9 273–277. $\endgroup$ – David Oct 10 '19 at 14:29
  • $\begingroup$ Counting the bases is meaningless. Eukaryotes possess non-conserved expansion segments that account for virtually all the variations in length. You can take ribosomes from a prokaryote, and insert them into eukaryotes, and they still function. I stand by the fact that they are virtually identical. Saying that they are related is obvious, but seems to indicate a lack of understanding for how genes evolve following their divergence after speciation. At the time lineages split, the genes are identical. Then the nucleotides change over time, but the structure remains. $\endgroup$ – Karl Kjer Oct 11 '19 at 11:25
  • $\begingroup$ I didn't want to answer myself and inevitably overlap with yours, so I suggested you edit it to clarify the point. But I have no wish to argue the point, I'll just write one of my own when I have the time. $\endgroup$ – David Oct 11 '19 at 12:11

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