Biology is not only a body of knowledge. It reshapes how long people live, what they eat and how societies make some of their hardest decisions. This final level looks at what biology has made possible, what has gone wrong, and the questions it now puts to citizens.

Germs

For centuries many doctors blamed disease on miasma, or bad air. In 1847 the Hungarian physician Ignaz Semmelweis noticed that women giving birth in a Vienna ward run by doctors died far more often than those in a ward run by midwives. Doctors often came straight from dissecting corpses. When Semmelweis required them to wash their hands in a chlorine solution, deaths among new mothers fell dramatically. Many colleagues rejected his findings, and he died in an asylum in 1865.

In the 1860s Louis Pasteur showed that microbes cause fermentation and spoilage, and the German physician Robert Koch identified the bacteria behind anthrax in 1876 and tuberculosis in 1882. Germ theory, the idea that many diseases are caused by microorganisms, transformed medicine and public health.

Vaccines and antibiotics

In 1796 the English doctor Edward Jenner noticed that milkmaids who caught mild cowpox seemed protected from smallpox. He infected a boy with cowpox and showed he resisted smallpox: the first vaccine. Nearly two centuries later, a worldwide vaccination campaign succeeded, and in 1980 the World Health Organization declared smallpox eradicated, the only human disease ever wiped out.

In 1928 the Scottish bacteriologist Alexander Fleming noticed that a mold contaminating one of his dishes had killed the bacteria around it. The substance it made, penicillin, was developed into a mass-produced drug during the Second World War. Antibiotics have saved countless lives, but as Level 5 showed, bacteria evolve resistance, and Fleming himself warned that misuse would speed the process.

Editing genes

In the 1970s scientists learned to cut and join DNA from different organisms. By 1982 bacteria engineered with a human gene were producing human insulin for people with diabetes. A far more precise tool arrived in 2012, when Jennifer Doudna and Emmanuelle Charpentier showed how to use CRISPR, a system adapted from a defense bacteria use against viruses, to cut DNA at almost any chosen spot. They shared the Nobel prize in chemistry in 2020. In 2023 regulators in Britain and the United States approved the first CRISPR-based treatment, for sickle cell disease.

A key distinction runs through the ethics of gene editing. Somatic editing changes cells in one patient's body and is not passed on. Germline editing changes eggs, sperm or embryos, so the change is inherited by future generations. In 2018 the Chinese scientist He Jiankui announced the birth of twin girls whose embryos he had edited. Scientists around the world condemned the experiment as unsafe and unethical, and he was sentenced to prison in China. Some argue germline editing may one day justly prevent serious inherited disease; others fear unknown risks, the use of editing to “enhance” children, and a future in which only the rich can afford it. The children affected cannot consent.

Genetically modified crops

Engineered crops, such as corn that resists insects, are widely grown. A major 2016 report by the US National Academies of Sciences, Engineering, and Medicine found no substantiated evidence that approved genetically engineered foods are less safe to eat than conventional ones. Critics raise other concerns: the control of seeds by a few large companies, weeds that evolve resistance to herbicides, and effects on small farmers and biodiversity. Both the safety evidence and these social questions deserve a hearing.

What biology says about race

Any two humans share about 99.9 percent of their DNA. In 1972 the geneticist Richard Lewontin found that most human genetic variation lies within populations, not between them; later studies broadly agree. Ancestry is real and sometimes matters in medicine, as with sickle cell disease. But traditional racial categories capture only a small share of human variation, and Africa, where our species arose, holds more human genetic diversity than any other continent.

History shows what happens when biology is bent to serve prejudice. The term eugenics was coined by Francis Galton in 1883. In the following decades, tens of thousands of Americans were sterilized against their will, a practice the US Supreme Court upheld in Buck v. Bell in 1927, and Nazi Germany carried eugenics to mass murder. Equal human worth is a moral principle, not a biological finding, but nothing in biology undermines it.

Worked example: reading a gene-editing headline

Suppose you read: “Scientists Edit Genes to Cure Inherited Blindness.” Questions from this course help you judge it.

• Somatic or germline? Is the edit made in a patient's eye cells, or in embryos whose children would inherit it? The ethics differ greatly.
• What is the evidence? Was there a control group? Has the study been peer reviewed, and how many patients took part?
• Treatment or cure? Did vision improve for a while, or permanently? Early results are often modest.
• Who gets access? Gene therapies can cost millions of dollars per patient. Who will pay?

Asking these questions does not mean distrusting science. It means taking it seriously enough to read it well, which is the habit this whole course has aimed to build.