Conceiving conception

VIRTUALLY EVERY adult on the face of the planet knows how babies are made, but this was not always the case. For most of humanity’s history, the only certainty was that male semen was necessary for conception, but no one could be sure why. The way we found out the truth reveals some surprising things about the nature of scientific knowledge.

The first great breakthroughs came in the 17th century. In 1667, the Danish physician Nicolaus Steno argued that all female animals (including women) produce eggs. Five years later, two of Steno’s university friends from the Netherlands, Jan Swammerdam and Reinier de Graaf, claimed that the egg was located in tiny vesicles within what is now called the female ovary.

In 1677, another Dutchman, Antoni Leeuwenhoek, sent an amazing report to the Royal Society of London describing what he saw when he turned his primitive but powerful single lens microscope onto that most mysterious of substances: male semen.

Less than “six beats of the pulse” after ejaculating, Leeuwenhoek put his microscope to his eye and was amazed to see hundreds of thousands of tiny wriggling “eels.” He soon correctly concluded that they were the male contribution to the generation of life.

To the modern eye, this should have been the end of the story. Within the space of a few years, the two complementary components of reproduction had been discovered. Humanity should have realized that each contained half of a potential human being and that the two components fused during fertilization.

Instead, for more than 150 years, scientists were divided into two camps -- “ovists” and “spermists” -- each arguing that their favored component was essential. For the ovists, the semen merely awoke the egg; for the spermists, the egg was simply food for the sperm. Strikingly, most ovists argued that the sperm were parasites living in the life-enabling semen and classified them as parasitic worms.

There were two interconnected reasons why people did not immediately understand that egg and sperm were equivalents. One reason was that everything about them seemed to be different. For example, in chickens and other animals people were familiar with, the egg was single, huge and apparently inert, whereas there were hundreds of millions of minute, wriggling sperm.

The second reason tells us something about the way science develops. Put simply, thinkers could not understand what they saw because they did not have the right ideas to interpret their observations. People could not realize that egg and sperm were equivalents because they did not believe that both parents contributed equally to the offspring.

The complementary roles of the two sexes seem obvious to us because we know not only about genetic heredity but also understand the mechanism, DNA, through which each parent transmits characteristics to its offspring. Amazingly, it was only in the 1830s that scientists even realized there was such a thing as heredity. Think about it: Sometimes children look like a mixture of their parents (skin color), sometimes they look like one, or like a grandparent (eye color), and sometimes they look like neither. If you simply look at family resemblance, there is no obvious pattern, no apparent force you could imagine investigating or even giving a name to.

The realization that each parent transmits something to the offspring began to surface by the end of the 18th century. Pedigrees of inherited diseases show that these traits are inherited down the generations, and, by the 1830s, French physicians had adopted a legal term to describe this phenomenon: l’heredite.

Around the same time, the English farmer Robert Bakewell did something amazing. Using selective breeding on a huge number of animals, he was able to create the New Leicester breed of sheep, which grew to full size in two years rather than four. All this work convinced thinkers that something passed from parents to offspring.

The final step was made in the 1840s. Once it was understood that all life is made up of cells, it became apparent that, despite their differences in size and motility, egg and sperm are indeed equivalent. Within a few years, new microscopic techniques made it possible to observe fertilization -- the fusion of egg and sperm.

Apart from its intrinsic interest, this story has intriguing implications. The 17th century discoveries of egg and sperm were widely accepted and had an enormous impact, but their true significance remained hidden for 150 years because thinkers could not interpret them correctly.

How many of today’s ideas will turn out to be similarly misunderstood? That is what makes science so exciting and progressive. Today’s certainties may be reinterpreted in the light of tomorrow’s discoveries.