by Matthew Cobb

Today’s Google doodle (above) is in honour of Nicolas Steno (1638-1686) – it would be his 374th birthday today (in fact it’s a bit more complicated than that, because he was actually born on 1 January 1638, but under the old Julian calendar…).

The doodle fetes Steno’s principle of superposition, which is the idea that, in any geological strata, the lower layers are older than the upper layers. Furthermore, it shows fossils in the rocks – Steno was the first person to clearly show that fossils were actually the remnants of long-dead animals.

But Steno was not just the father of geology. He was one of the most amazing thinkers who participated in the Scientific Revolution that took place in the 17th century. He also made lasting contributions to anatomy and physiology, and above all to our understanding of where we come from. All in the space of about 12 years.

Between 1662 and 1667, in Amsterdam, Leiden, Paris and Florence:

He discovered the duct that takes saliva from the parotid gland to the mouth – this is still called ‘Steno’s duct’.

He made the first scientific dissection of the human brain.

He showed how muscles work.

That would be enough for anyone. But Steno’s big breakthroughs came after his 1667 book on muscles (Elementorum Myologiae Specimen) had been approved by the Holy Office (the church censor). Just before it went to press, Steno added two brief pieces to his book, both of which had the same origin: the dissection of a shark.

In October 1666, French fishermen landed a gigantic great white shark at the port of Livorno. They weighed it (1200 kg), took out its liver, hacked its head off and then rolled the rest into the sea. The head was then brought to Florence for Steno to dissect in front of the Duke Ferdinand’s court.

Steno noticed that the sharks’ teeth looked remarkably like glossopetrae (tongue-stones) which could be found on exposed rocks in the region, and which were thought to be vipers’ tongues. Like a number of previous thinkers, Steno suggested that glossopetrae looked like sharks’ teeth because that is what they were. His dramatic drawing (in fact of another shark) shows the teeth:

The problem, of course, was how they got into rocks on the top of mountains.

Steno was a good Christian – at this stage he was still a Protestant – and he had a simple answer: the flood. Fish, like sharks, would have been stranded on the top of mountains when the waters receded. He also pointed out that during earthquakes, huge bits of land could move up or down, and that over time, this might also explain how the remains of marine organisms could be found at high altitudes.

Now Steno didn’t have any idea of deep time – if he thought about how old the world was, I assume he would have agreed with something like Bishop Ussher’s view that it was all in the Bible, and so around 6,000 years old. And he was also wily enough to know that his suggestion could be a problem for the Churh, so he used Galileo’s device of claiming that the view he had outlined was merely one possibility amongst many:

‘While I show that my opinion has the semblance of truth, I do not maintain that holders of contrary views are wrong. The same phenomenon can be explained in many ways; indeed Nature in her operations achieves the same end in various ways. Thus it would be imprudent to recognise only one method out of them all as true and condemn all the rest as erroneous.’

In the final part of Elementorum Myologiae Specimen, entitled Historia Dissecti Piscis ex Canum Genere (Study of the dissection of a dogfish) – which is a mere nine pages long – Steno described the dissection of a small female dogfish that gives birth to live young. Most of this is is pretty unexceptional, and then in the final couple of pages, Steno used an a simple analogy and, in a few lines, made a huge break-through in humanity’s understanding of ‘generation’ – where animals come from, and in particular the role of the female ‘testicles’ (what we would now call ovaries).

First he noted that much of the internal anatomy of this shark was very similar to that of an egg-laying ray that he had previously dissected. Then he went on to muse about the nature of ‘generation’ in oviparous and viviparous animals, before coming to this amazing conclusion:

‘having seen that the testicles of viviparous animals contain eggs and having noticed that their uterus opened into the abdomen like an oviduct, I have no doubt that the testicles of women are analogous to the ovary, whatever the manner the eggs themselves, or the matter that they contain, pass from the testicles to the uterus.’

‘The testicles of women are analogous to the ovary’: in other words, women have eggs. This amazing statement – almost a throwaway comment in a brief section on sharks – was the start of our modern understanding of both human reproduction, and on the essential unity of the animal kingdom.

Over the next couple of years, Steno found ovaries in deer, guinea pigs, badgers, wolves, asses and mules, but he never published anything further on the question.

Four years later, two of Steno’s old student friends, Jan Swammerdam and Reinier de Graaf, were slugging it out in public over who had been the first to discover that women have eggs – Swammerdam did some neat dissections, de Graaf did some neater experiments. The Royal Society of London was called in to adjudicate the matter. It took them so long that by the time they issued their verdict, de Graaf was dead, and Swammerdam and Steno had both become obsessed with religion (Swammerdam went all mystic, Steno became a devout Catholic and ended up a bishop; both men abandoned science because of their beliefs). And the Royal Society rightly gave the credit to Steno – the man who discovered that women have eggs.

Five years later, our understanding of what is going on in ‘generation’ became even more complex when Antoni Leeuwenhoek, an uneducated Dutch draper who had known de Graaf, discovered spermatozoa. But for reasons that will have to be dealt with at another time, it would not be until 1827 until von Baer actually saw a human egg, and not until the 1850s that it was realised that egg and sperm were complementary halves of the future organism, and that both were necessary for life to arise.

Google’s doodle rightly commemorates Steno’s principle of superposition. I would like to have seen some eggs floating around in the doodle, too. Without Steno’s brilliant insight, we would not have discovered what we know in the same way, or at the same pace. Maybe they can include a shark and an egg next year.

If you want to know more about Steno or about the discovery of the human egg, the best place to start is either of these two books:

Matthew Cobb (2007) The Egg & Sperm Race (published in the US as Generation)

Alan Cutler (2003) The Seashell and the Mountaintop