text: There are many examples of ancestral proteins that have been computationally reconstructed, expressed in living cell lines, and – in many cases – purified and biochemically studied.
  * The Thornton lab notably resurrected several ancestral hormone receptors (from about 500Ma) and collaborated with the Stevens lab to resurrect ancient V-ATPase subunits from yeast (800Ma).
  * The [https://zebra.berkeley.edu/ Marqusee] lab has recently published several studies concerning the evolutionary biophysical history of E. coli Ribonuclease H1.
  Some other examples are ancestral visual pigments in vertebrates, enzymes in yeast that break down sugars (800Ma); enzymes in bacteria that provide resistance to antibiotics (2 – 3Ga);  the ribonucleases involved in ruminant digestion; the alcohol dehydrogenases (Adhs) involved in yeast fermentation(~85Ma); and RuBisCO in Solanaceae.
  The age of a reconstructed sequence is determined using a molecular clock model, and often several are employed. This dating technique is often calibrated using geological time-points (such as ancient ocean constituents or BIFs) and while these clocks offer the only method of inferring a very ancient proteins age, they have sweeping error margins and are diffuclt to defend against contrary data. To this end, ASR age' should really be only used as an indicative feature and is often surpassed altogether for a measurement of the number of substitutions between the ancestral and the modern sequences (the fundiment on which the clock is calculated). That being said, the use of a clock allows one to compare observed biophysical data of an ASR protein to the geological or ecological environment at the time. For example, ASR studies on bacterial EF-Tus (proteins involved in translation, that are likely rarely subject to HGT and typically exhibit Tms ~2C greater than Tenv) indicate a hotter Precambrian Earth which fits very closely with geological data on ancient earth ocean temperatures based on Oxygen-18 isotopic levels. ASR studies of yeast Adhs reveal that the emergence of subfunctionalized Adhs for ethanol metabolism (not just waste excretion) arose at a time similar to the dawn of fleshy fruit in the Cambrian Period and that before this emergence, Adh served to excrete ethanol as a byproduct of excess pyruvate. The use of a clock also perhaps indicates that the origin of life occurred before the earliest molecular fossils indicate (>4.1Ga), but given the debatable reliability of molecular clocks, such observations should be taken with caution.