People have always been interested in how a child comes to look like other members of its family. However, scientists did not discover DNA, genes and chromosomes until relatively recently. But that did not stop people in earlier times from taking a guess about how heredity works. For the most part, their beliefs came from interpretations of what they saw. In other words, their conclusions were not tested using the scientific method; they were based only on untested observations.

Pythagoras (ca. 580–500 b.c.), a Greek philosopher and mathematician, for example, thought that all the necessary hereditary material came from the father, and that the mother only provided a place and food for the baby to grow. He also believed that this hereditary material was gathered from all over the father’s body to pass on to his offspring.

However, Pythagoras’s theory did not explain why a baby might have dad’s nose but also have mom’s hands. So Empedocles (ca. 490–430 b.c.), another Greek philosopher, explained this lapse in Pythagoras’s theory by saying that both mom and dad possessed hereditary material that was passed on to their offspring through sexual fluid.

Building on the ideas of Pythagoras and Empedocles, Aristotle (384–322 b.c.) believed that while, in fact, both mother and father did contribute hereditary material to the unborn baby, it was not due to the sharing of sexual fluids, but to the sharing of blood. Aristotle believed that semen was a man’s purified blood. When this purified blood commingled with the menstrual blood in a woman, the result was a child that shared traits with both mom and dad. Some scholars believe that Aristotle’s thoughts are where the terms such as blue blood and bloodline come from when people speak of hereditary traits.

It took almost 2000 years to shake off Aristotle’s idea. William Harvey (1578–1657), an English physician who mainly studied the heart and circulatory system and explained how the beating of the human heart circulates blood throughout the body, also studied reproduction, primarily in chickens and deer. Through his research, Harvey came to understand that menstrual blood had nothing to do with the formation of a baby. Harvey thought that human babies were made much the same way chicks were, by the fusion of sperm and eggs. It turns out that Harvey was right, but it would take more than 200 years to prove it.

Twenty years after Harvey died, Dutch scientist Antonie van Leeuwenhoek (1632–1723) successfully invented a microscope that could reveal the sperm in semen. Leeuwenhoek’s invention sparked the idea that all living things arise from interactions between cells. This idea helped to dispel a popular notion that had been around since the time of Aristotle—the theory of spontaneous generation. People, including scientists, believed spontaneous generation could occur, or that living things could arise spontaneously from inanimate objects.

People who believed in spontaneous generation thought, for example, that flies could arise from a piece of meat. They believed this because they observed that meat left on a kitchen counter was quickly swarmed with flies. Therefore, they believed that the flies must have been spontaneously born from the meat. There was other “proof,” too. Every year, the Nile River flooded. Along with the floodwaters, the overflowing river brought fertile soil and frogs to the land. These frogs did not exist when the land was drier. This gave people the idea that frogs must come from mud. Another observation that led people to believe that the theory of spontaneous generation was true was the number of mice in barns that contained moldy grain. Because mice were almost always found where there was moldy grain, people believed that the mice actually came from the moldy grain.

Although Leeuwenhoek’s microscope helped put this idea of spontaneous generation to rest, it was the experiments of an Italian physician named Francesco Redi (1626–1697) that truly disproved the idea. In 1668, Redi set up an experiment by putting pieces of meat into several wide-mouth jars. Redi’s hypothesis stated that if flies came from meat, all of the jars in the experiment should have maggots (fly larvae) and flies in them by the end of the experiment. For his control group, Redi allowed several of the jars with meat in them to stay open to the air. These jars simulated the conditions meat would encounter in a butcher shop of that time period. For his experimental groups, he prepared several other jars that contained meat and divided them into two groups. Redi covered the mouths of the jars in one group with gauze and the other group with lids. Then he waited. Each day, he counted and recorded the number of maggots or flies around each jar in each group.

Redi observed flies in and around the jars that were left open to the air. Soon maggots appeared on the meat in those jars, and then there were more flies than ever. While Redi never observed flies inside the jars covered with gauze, he did see flies sitting on the gauze at the mouth of the jars. Eventually some maggots were seen inside these jars. No flies or maggots were ever observed in the jars sealed with a lid.

Redi concluded that in the jars open to the air, flies entered the jars and laid eggs on the meat, and then the eggs hatched and the maggots emerged. These maggots eventually turned into adult flies. Even though flies could not enter the jars covered with gauze, they could lay eggs on the gauze. Redi also deduced that either the eggs fell through the gauze onto the meat and then hatched, or the eggs hatched on the gauze and the maggots fell through and onto the meat. Either way, the eggs and then the maggots came from flies that were on the outside of the jar. Because there were no flies inside the jars sealed with a lid, they could not lay eggs and there were no maggots. From these observations, Redi concluded that flies do not arise from meat—only flies can make other flies.

Redi’s experiment disproved spontaneous generation, using three different jars to show that flies could not spontaneously arise from meat. The jar with the lid prevented flies from laying their eggs on the meat, and so no maggots (which would turn into fl ies) appeared. The open jar and the jar with gauze allowed for eggs to be laid onto the meat or on the gauze, respectively.

One would think that this would have been the end of the spontaneous generation debate, but it was not. People were willing to believe that larger organisms, such as flies and humans, could not be produced by spontaneous generation and, in fact, needed parents. But with the better microscopes of the 1600s, people were seeing all sorts of bacteria, yeast, and other single-celled organisms. They did not know where these tiny organisms came from, but they always seemed to be present in spoiled broth. So it seemed logical to people at the time that these microorganisms spontaneously generated in spoiled broth.

In 1745, a Scottish clergyman and naturalist named John Needham (1713–1781) claimed that all inorganic molecules, such as air and the oxygen in it, contained a “life force” that allowed spontaneous generation to occur. Needham proved his claim by boiling some broth (to kill any microorganisms already present in the broth), pouring it into “clean” flasks, and capping the flasks with natural cork stoppers. When microorganisms still grew in the broth, Needham was convinced that his conclusions about a “life force” were correct. Not everyone was convinced, however. A few years later, Lazzaro Spallanzani (1729–1799), an Italian biologist, tried Needham’s experiments again. Spallanzani boiled some broth for an hour. Then he took some of the boiled broth and sealed it in several glass flasks by melting the mouths of the flasks shut. Other portions of broth were put into flasks capped with natural cork stoppers, just like in Needham’s experiment. For his last experimental group, Spallanzani boiled broth for only a few minutes, put it into flasks, and melted the mouths of the flasks shut.

Spallanzani found that the only flasks that did not contain microorganisms were the flasks containing broth that had been boiled for an hour and then sealed by melting the mouths of the flasks shut. From these results, Spallanzani was convinced that microorganisms could not be spontaneously generated, either. Needham, however, was not convinced. He believed that the microorganisms could not be spontaneously generated in Spallanzani’s sealed flasks because the “life force” was destroyed during the extensive boiling time, and that it could not enter the flask after it had been sealed. This created such a big argument between the two scientists that the Paris Academy of Sciences offered a prize to the first person who could develop and carry out an experiment to put the argument to rest.

In 1859, Louis Pasteur (1822–1895) claimed the prize. In his experiment, Pasteur also boiled broth to kill all of the bacteria in it. But he boiled the broth in different-shaped flasks. He then allowed the flasks, containing the sterilized broth, to cool. The flasks in the control group had straight necks, open to the outside air and any bacteria in it. Other flasks had thin, S-shaped necks, called swan-neck flasks; they would allow air and, therefore, the “life force” that Needham insisted needed to be present, to enter the flasks—but bacteria would settle out, due to gravity, on the necks of the flasks and not reach the broth. Pasteur found that broth in the straight-necked flasks spoiled, while broth in the swan-necked flasks did not, even though fresh air could get into the flasks. This finally put to rest the idea of spontaneous generation.

Preformation was another idea that many intellectuals held during the seventeenth and eighteenth centuries. Preformationists believed that humans grew from tiny, fully formed humans that already existed in either the egg or the sperm. The miniature human was called a homunculus. Dutch scientist Jan Swammerdam (1637–1680) believed that the miniature human was encapsulated in a man’s sperm and that the mother was only a vessel for development of the fetus. But then, a century later, Swiss scientist Charles Bonnet (1720–1793) said, no, the miniature human was actually in the mother’s egg. In 1745, however, French mathematician and biologist Pierre-Louis Moreau de Maupertuis (1698–1759) argued that preformation would not explain the existence of babies born with birth defects. Instead, Maupertuis proposed that both parents had “particles” within their hereditary material that, when commingled, formed a baby. Maupertuis also believed that sometimes one parent’s “particles” had a stronger influence over the development of the baby than the other parent’s “particles.” This was the precursor to the modern idea of dominant and recessive genes.

Charles Darwin (1809–1882), an English naturalist, is probably best known for his ideas about evolution and natural selection. Darwin’s theory of natural selection states that those individuals in a population who carry the most advantageous traits will have a better chance at survival and reproduction. Therefore, these advantageous traits are passed on to offspring generation after generation. In this way, these traits are increased in the population. Darwin believed that every organ and tissue in the body produced small particles, which he called “gemmules,” that flowed through the body’s bloodstream. He explained that these gemmules, which converged into sperm and egg that merged during fertilization, and “pangenes,” which made up the hereditary material, were passed down to the offspring. This theory was called the pangenesis theory. Early on, Darwin also believed that a parent’s pangenes could change throughout their lifetime. This would mean that offspring could receive traits that had changed during the parent’s lifetime. Darwin later abandoned this part of his theory.