It is an often-repeated claim that human, and in fact all animal blood is salty because we evolved from aquatic organisms, and that blood has a similar concentration of salts as ocean water, or at least as primordial ocean water.

Is any of that true? Does science know what the salinity of the oceans was billions of years ago, and was it indeed similar to the blood of today's organisms? Does that knowledge prove anything about evolution and the origin of life?

I am wondering, because I tried to Google for the answers, and unfortunately all I could find were creationists referencing each other how it couldn't possibly be true because evolution is a lie in the first place etc., and they claim that there is no empirical research that would compare salinity levels of organims' cells and ocean water.

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    $\begingroup$ 'blood is salty because oceans are salty, and life began in oceans' is a weird way of putting it. Can you cite non-creationist sources that put it this way? $\endgroup$
    – bukwyrm
    Oct 17, 2019 at 12:24
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    $\begingroup$ Do you have any citation for this claim being "often-repeated"? I've spent quite a bit of time studying literature on both sides of the evolution and creationism debate, and I can't remember ever seeing this argument having been made. I'm quite curious where you ran across it and who is promoting it. It is quite absurd, so I'm especially curious if it's some kind of disinformation propaganda by anti-evolution activists. $\endgroup$ Oct 18, 2019 at 4:03
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    $\begingroup$ Also note that freshwater fish actively retain salt, and ocean fish actively try to excrete salt. This means that animals are quite able to maintain a desirable salinity even if that's different from our surroundings. This basically means that even if the levels had been the same, it'd have proven nothing at all. $\endgroup$
    – Gloweye
    Oct 18, 2019 at 6:56
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    $\begingroup$ @Hackworth That's a very weird link. There's a claim that ocean salinity is similar to blood salinity, but a few paragraphs later he says the (modern) ocean salinity is four times larger. The follows the claim that early oceans were this "quarter the salinity of modern oceans", but without any reference. Indeed, there's no references in the article at all, which should already be suspicious, and I couldn't find any references to a related paper or much about the author either. And as already noted, I never saw anyone claiming this is a proof life came from oceans, except from creationists. $\endgroup$
    – Luaan
    Oct 18, 2019 at 8:02
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    $\begingroup$ FWIW, I recall hearing the same claim in my high school health class nearly 20 years ago (in a classroom video that seemed old back then). Not that necessarily qualifies as "often-repeated", but anecdotal evidence of the claim dating back a while. (Though I'm not sure how that fits into the creationist/evolution "debate", I'd think evidence that life began in oceans would run counter to the standard creationist viewpoint, no?) $\endgroup$
    – PGnome
    Oct 18, 2019 at 15:37

3 Answers 3


Short answer
Early sea water had a very different osmolality than blood plasma.

The reference range of serum osmolality is 275–295 mosm/kg (mmol/kg) (MedScape). The osmolarity of sea water is about 1000 mOsm/l (Wikipedia), but it can vary substantially between different seas, namely between 642 and 1,480 mOsm/kg (Ninawe & Banik, 1998). Nonetheless, the range of osmolalities are substantially higher than that of serum. That's why we cannot consume sea water, without losing more water than gaining it (NOAA). According to Astro Biology Magazine, the salinity of early oceans 3.5 billion years ago, when life first developed, was even higher. So popular accounts that they are the same and that this has important evolutionary consequences are likely far-fetched.

- Ninawe & Banik, J. Mar. Biol. Ass. India (1998); 53(2): 230-36

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    $\begingroup$ Do you have any idea why the oceans were MORE salty in the past? I thought they are salty because water brings salt to them from surface and then evaporates, so I naturally expected them to be less salty in the past. This really surprised me. $\endgroup$ Oct 18, 2019 at 9:16
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    $\begingroup$ I'd think that the salinity or osmolality of the oceans when tetrapods' ancestors first started venturing on to land (around 400 million years ago) would be a lot more important to the argument than what it was when life first developed. $\endgroup$ Oct 18, 2019 at 13:37
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    $\begingroup$ I think the two pieces this answer is missing are a) Some reference to the first animals, which were likely osmoconformers (since modern sponges and cnidarians are), and b) Relative ion concentrations and the consistency of high extracellular sodium/high intracellular potassium, which user1136's answer touches on: this is the way that blood and ocean are "similar" even if total osmolarity is not. $\endgroup$
    – Bryan Krause
    Oct 18, 2019 at 15:30
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    $\begingroup$ Just a caution for anyone who is tempted to directly compare the numbers provided, you will have to first change units. The blood concentration is given in mOsm/kg, and the ocean concentration in mOsm/L. Since these are salty solutions more concentrated than pure water, it would not be appropriate to use the density of water and say 1 L = 1 kg. The density of the actual solution would have to be used. However, the orders of magnitude of these concentrations are probably still appropriate for direct comparison and lead to the same conclusion. $\endgroup$ Jan 23, 2022 at 5:13
  • $\begingroup$ @electronpusher - good point. I should've converted those numbers. Can attempt to correct them in the future. $\endgroup$
    – AliceD
    Jan 24, 2022 at 12:23

The argument that blood plasma resembles sea-water, in essence, relies on the notable similarity in concentration of two ions in plasma and sea-water, compared with their intracellular concentration: sodium and potassium.

  • The concentration of sodium-ion in sea-water is about 450 mM, whereas the concentration of potassium-ion is only about 10 mM (ref)
  • The plasma concentration of sodium-ion is about 140 mM and the concentration of potassium-ion is about 3-5 mM (ref, but many clinical biochemistry sources will confirm)

That is, both plasma and sea-water are 'high-sodium, low-potassium' and the concentrations are remarkably similar. If we contrast this with the intracellular concentration of these ions:

  • The intracellular concentration of potassium-ion is about 150 mM, whereas the concentration of sodium-ion is about 10-20 mM (ref)

That is, the 'intracellular milieu' is 'high-potassium, low-sodium'. Blood plasma, of course, is considered an extracellular fluid.

We can generalize further and say (Skou, Nobel lecture):

  • The intracellular concentration of potassium-ion is about 150 mM whereas extracellular concentration is about 4 mM.
  • The intracellular sodium-ion concentration is 10-20 mM whereas the extracellular concentration about 140 mM

Thus sea-water, in terms of sodium-ion and potassium-ion concentrations, resembles the extracellular milieu, leading to the somewhat hyperbolic view that "blood can also be thought of as a private ocean...".

But the similarities are striking.

At one time it was thought that this concentration difference was due to cells being impermeable to sodium-ion, a hypothesis first put forward by E.J. Conway.

It is it now known, of course, that concentration imbalance is maintained by the sodium-potassium pump, which is present in animal cells but not in plant cells (W.D. Stein). The presence of this pump allows animal cells to be in osmotic equilibrium (no change in osmotic pressure) with their surroundings. (Plant cells, in contrast, have developed a cell wall that allows high internal osmotic pressures and which may (to again quote Stein) "have condemned [them] to a largely sessile life")

The history of its discovery (and much more) is given in The Identification of the Sodium Potassium Pump, the Nobel lecture by J.C. Skou

Addendum 1: Historical Origins

According to a review by Mulkidjanian and colleagues, it was Macallum in 1926 that first drew attention to the similarities in inorganic composition of sea-water and plasma and its significance as evidence that the first animals emerged from the sea.

I think R.L. Berg (see Addendum 3), at a famous conference in 1957, also drew attention to this, but I do not have access to the reference.

But just because animals emerged from the sea does not necessarily mean that life began in the sea. Mulkidjanian and colleagues point to the fact that "inorganic composition of the cell cytosol dramatically differs from that of modern sea water" (which was also first pointed out by Macallum in 1926) may indicate that life began in a high-potassium, low-sodium environment and that life in the sea only became possible after the evolution of the sodium-potassium pump allowed organisms to maintain their 'fundamental biochemical architecture' (the chemical conservation principle).

These authors suggest condensed geothermal vapours as a likely environment for life's origin but not everone agrees.

I notice from the comments that the similarity and blood plasma and sea-water is considered somewhat heretical and perhaps not sufficiently backed up by evidence. This viewpoint has a long history in 'mainstream' science.

Pearse in The migrations of animals from sea to land (1936), for example, has this to say:

The general similarity of the bloods of animals to seawater has been interpreted as indicating that all animals originated in the sea. ... While there are many discrepancies to be explained, it is a common belief that "blood is modified seawater" (quoting Pantin,1931).

Another good source with some great references is The Oceans, Their Physics, Chemistry, and General Biology (1942) by Sverdrup, Johnson and Fleming, which is available as an e-book, especially the section in Chapter VIII on Sea Water and the Body Fluids.

Finally, there is nothing 'creationist' about the above arguments, but creationists, in the main, don't like them. See Is the Sodium Chloride Level in the Oceans Evidence for Abiogenesis? by Bergmann, for example.

Although I do not agree with any of the views expressed in this article (especially the view that the Flood "likely carried to the earth’s surface many tons of pulverized crustal material containing large amount of salts and other minerals"), it contains many good references to the 'mainstream' point of view (which the author attempts to debunk).

Addendum 2: Serum and Plasma

Plasma is liquid component of blood that is clinically obtained as the supernatant (upper 'layer') generated by centrifuging blood collected in the presence of an anti-coagulant such as heparin or EDTA.

Serum is plasma without clotting factors and is clinically obtained by first letting blood clot, centrifuging, and then collecting the supernatant.

Both serum and plasma are considered extracellular fluids, in contrast to cytoplasm, which is the intracellular fluid that 'holds' the organelles and other cellular components.

There is no clinical difference in serum and plasma sodium levels, but serum concentrations of potassium are somewhat lower (about half-millmolar or thereabouts) than those of plasma (ref)

Addendum 3

It appears that the R.L. Berg quoted above is Raisa L Berg, the eminent Russian Darwinist and geneticist who, among many other achievements, wrote In Defense of Timoféeff-Ressovsky.

As I stated above, I do not have access to the original reference but R.L Berg is quoted in Bergman's article (also quoted above) as follows:

Berg (1959, pp. 169–170) notes, “The most important arguments in favor of the hypothesis that life originated exclusively in the ocean” are, first, “the similarity between the salt composition of the body fluids of land animals and that of the waters of the ocean.”

He [sic] then adds that the “similarity in the salt composition of the waters of the ocean and that of the body fluids of land animals could be accounted for” only by evolution (Berg 1959, p. 170).

It would be very nice to have her views recorded 'first hand' here (the OP question has generated a lot of interest).

Perhaps someone with access to the article could post a separate answer?

Addendum 4

Adrian (in a comment) has drawn attention to the following quote, from the best-selling book The Sea Around Us by Rachel Carson, first published in 1951:

"Fish, amphibian, and reptile, warm-blooded bird and mammal - each of us carries in our veins a salty stream in which the elements sodium, potassium, and calcium are combined in almost the same proportions as in sea water."

  • $\begingroup$ Congratulations on an excellent answer. I am travelling at the moment, but will see if my University library has access to the article on my return, although if it is in Russian I won’t be able to translate it myself. $\endgroup$
    – David
    Oct 21, 2019 at 13:37
  • $\begingroup$ I am having trouble in tracking down the the paper of R.L Berg in First International Symposium of The Origin of Life on the Earth, Moscow, August 19-24, 1957. My University Library does not have a copy or online access, and they won't process my Inter-Library Loan request (The British Library will have a copy) because I don't know the title and page numbers for them to photocopy. Apologies. $\endgroup$
    – David
    Nov 13, 2019 at 11:59
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    $\begingroup$ I found this answer after reading a passage in The Book of Eels, by Patrik Svensson, which references a passage in Rachel Carson's The Sea Around Us (see goodreads.com/work/quotes/2423508-the-sea-around-us, "each of us carries in our veins a salty stream..."), so this idea has been mentioned in popular science books for several decades now. $\endgroup$
    – Adrian
    Jan 19, 2022 at 23:50
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    $\begingroup$ This is the link to the report by Berg in the symposium on "The Origin of Life on the Earth": doi.org/10.1016/B978-1-4831-9737-1.50023-0 (paywalled). Interestingly, just this chapter costs 22 Euros on Elsevier but I found the first volume of the proceedings for 14 Euros (the actual citation by the Bergman article; on this site: booklooker.de). $\endgroup$
    Jan 20, 2022 at 11:18
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    $\begingroup$ Great answer, very thorough. The quality we should all strive for. $\endgroup$ Jan 23, 2022 at 5:21

Even shorter answer: This can be easily (well, not that easily since you need to convert mols to milligrams) answered by looking at your last lab result.

Say you had Na: 135 mmol/ml on there which would be a plausible value. That's 135*22.88/1000 mg/ml or 3.1 g/l. Sea water has around 19 g/l, so that's a clear, definite "no".

It wouldn't be quite such a strikingly obvious "No!" if you had asked about osmolarity instead of salinity. Because unlike blood, oceans usally don't have significant amounts of plasma protein. The answer would still be a clear "No" (for non-fish animals, anyway), but not nearly as strikingly.


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