Lesson 10 — What If...?
What Is Evolution Really?
Learning Material
1 pagesLesson 10 — What If...?
Understanding the Complex: What Is Evolution Really?
Evolutionary biology is grounded in evidence — in fossils, genomes, direct experiments, observations of living populations. But some of its most powerful thinking is speculative: thought experiments that use what we know to probe what we don't, that push at the edges of the theory and reveal what it can and can't explain.
This lesson offers three such thought experiments. They're grounded in real science. But they're also deliberately open — the point is to think, not to arrive at settled conclusions.
What if we could reconstruct the last common ancestor of all animals?
Every animal alive today — every insect, fish, bird, mammal, worm — shares a common ancestor that lived somewhere around 600–700 million years ago. We call it the "urbilaterian," and we know almost nothing about what it looked like. It left no definitive fossils. What we know about it comes from comparing the genomes and development of living animals and inferring what their shared ancestor must have had.
The urbilaterian almost certainly had a mouth, a gut, and some kind of sensory system. It likely had the basic Hox gene toolkit that controls body plan development. Beyond that, we're speculating.
But here's the thought experiment: what if paleogenomics advanced to the point where we could reconstruct its genome? We've already begun to do this computationally — inferring ancestral gene sequences by comparing those of living descendants. If we had a sufficiently complete and accurate reconstruction, could we synthesize it? Could we build a living organism that approximates what lived 650 million years ago?
This is not currently possible. The computational challenge is vast — we're comparing sequences separated by hundreds of millions of years of divergence, and errors compound. The ethical questions are real — what are our obligations to a created organism? But the possibility raises a profound question about evolution itself: is the history of life deterministic? If we ran the tape again, starting from the same ancestor, would we get something like the same result?
Richard Lenski's LTEE provides a partial answer. When he traced back the citrate-metabolizing innovation in one of his flasks, he found that particular chance mutations earlier in the experiment had set the stage — without them, the later citrate-enabling mutation wouldn't have worked. Replay the tape from the start with the same organisms and you'd likely not get citrate metabolism. Small contingencies compound.
Evolution is not deterministic. The history of life is, in part, a story of frozen accidents.
What if antibiotic resistance makes all antibiotics useless?
This is not a thought experiment in the sense of being far-fetched — it's a risk that public health agencies consider serious and near-term. But playing it through helps illuminate what evolution means in a medical context.
Imagine a world where, over the next thirty to fifty years, resistance spreads to all major antibiotic classes. (We're not there yet — new antibiotic classes are still being developed, though the pipeline is thin and development is slower than resistance is emerging.) In such a world, routine surgeries become dangerous again: hip replacements, appendectomies, caesarean sections all carry high risk of untreatable infection. Chemotherapy becomes far more dangerous. Infections that are currently trivial become life-threatening.
The evolutionary response to this scenario is already being developed: phage therapy. Bacteriophages are viruses that infect bacteria, and they've been used in eastern Europe and the former Soviet Union for decades as an alternative to antibiotics. Western medicine is now taking them seriously. Crucially, phages evolve too — they can, in principle, evolve faster than bacteria evolve resistance, because their generation times are even shorter.
This is a case where intentionally deploying evolution — harnessing the rapid evolution of phages to outpace bacterial resistance evolution — may be our best response to the consequences of having inadvertently selected for that resistance in the first place.
What if gene drives altered human populations?
Gene drives work in sexually reproducing populations. They could, in principle, be applied to human populations. This thought experiment is deliberately uncomfortable.
Imagine a gene drive designed to eliminate a severe genetic disease — Huntington's, perhaps, or Tay-Sachs. The drive would spread the disease-preventing allele through a population over generations, reducing and eventually eliminating the condition. It sounds benign.
But gene drives require no consent from subsequent generations. The people who would carry the drive-modified genome in generation three or four never agreed to any intervention. And who decides which genetic variants count as "disease" and which count as "normal variation"? The boundary is not obvious: sickle cell trait confers resistance to malaria; genes associated with mood disorders are also associated with creativity and certain cognitive traits; the genetics of autism overlap substantially with the genetics of exceptional mathematical ability.
The history of eugenics — the 20th-century movement to "improve" human genetic traits through selective reproduction, which ended in forced sterilizations and, in Nazi Germany, mass murder — should make everyone cautious about framing genetic variation as a problem to be eliminated.
This thought experiment doesn't have a clean conclusion. It is meant to illustrate that evolution is increasingly something humans can act on — and that the ethical frameworks for doing so are nowhere near as developed as the technical capability.
Three thought experiments: a reconstructed ancestor, a post-antibiotic world, a gene-drive intervention in human populations. In each case, the evolutionary science is what generates the question. The answers require ethics, politics, and philosophy as much as biology.
Next lesson: What are you taking away? — Course close and reflection.
Reading time: approx. 9–10 minutes