From a Tiny Alga to the Human Brain

What could a tiny green alga swimming towards light possibly have to do with the human brain?

At first glance, almost nothing; yet a scientific journey that began with researchers trying to understand how a microscopic alga senses light, eventually led to a technology that allows scientists to control selected nerve cells with light and it subsequently changed the way brain can be studied. Recently, it earned science's highest recognition as the Nobel Prize in Physiology or Medicine 2026 has been awarded to Karl Deisseroth (Stanford University, USA), Peter Hegemann (Humboldt University, Germany) and Georg Nagel (University of Wurzburg, Germany) for discoveries leading to optogenetics.

Beyond the achievement itself, there is a wonderful story about how science actually progresses. Important discoveries do not always begin with attempts to cure a disease or invent a technology. Sometimes they begin with somebody becoming curious about something in nature and asking a seemingly simple question: How does this work? Questions may initially relate to seemingly unrelated fundamental biology, but can ultimately lead to important discoveries directly related to humans and other beings.

Following an alga towards the light

Chlamydomonas is a microscopic, single-celled green alga. Like many organisms, it can respond to its environment - in this case, to light. Scientists wanted to understand - how?

That investigation led Peter Hegemann, Georg Nagel and other researchers to proteins called channelrhodopsins. These proteins sit in the cell membrane and respond to light by allowing electrically charged ions to pass through.

To a non-scientist, this might sound like an interesting but rather specialised detail about the biology of an alga. But scientists saw another possibility. A friend of mine working at University of Frankfurt during 2003-04 told me of his memories from the time when the researchers from Prof. Nagel's team were conducting Electrophysiology experiments to understand the nature of these proteins and then figured these were light sensitive channels.

Nerve cells also communicate through changes in electrical activity. So what would happen if this light-sensitive protein could be placed inside a neuron? Could one, quite literally, switch a nerve cell on with light?

The question was intriguing and the idea worked.

Karl Deisseroth and colleagues demonstrated that channelrhodopsin could be introduced into organismal neurons and used to control their firing with extraordinary precision.

From this grew optogenetics - a combination of genetics and optics that allows particular populations of cells to be controlled using light. The implications were enormous.

Imagine trying to understand a complicated electrical circuit by watching it from outside. You may see which components become active, but that does not necessarily tell you what each one actually does. Now imagine being able to select one component, switch it on or off, and observe what changes.

Optogenetics gave neuroscientists something approaching that ability in the living nervous system.

Researchers could begin asking: Which neurons initiate a particular movement? Which circuits influence reward or fear? What happens when a particular neural pathway is activated or silenced? How do circuits change in neurological or psychiatric disorders?

An obscure question about how an alga responds to light had unexpectedly opened a new window into the brain.

A Fascinating C. elegans (worm) Connection

There is another interesting chapter in this journey.

In 2005, Georg Nagel and colleagues took channelrhodopsin-2 from Chlamydomonas and expressed it in selected cells of a tiny nematode called Caenorhabditis elegans. (Prof. A. Gottschalk from Goethe University Frankfurt also deserves due credit for this work published in Current Biology in 2005). When the protein was expressed in muscle cells, illumination caused the animal's muscles to contract. When it was placed in mechanosensory neurons - the neurons through which the worm normally responds to touch - light itself could trigger the behavioural response.

It was the first use of channelrhodopsin optogenetics to control behaviour in an intact animal.

There is something beautifully simple about that experiment. A protein from an alga had been transferred into particular cells of a worm. Light was shone on the animal - and the animal responded. Super exciting for a researcher!

The experiment helped demonstrate a principle that would subsequently become enormously powerful in neuroscience: genetically identify the cells you want to study, make them responsive to light, and then use light to ask what those cells actually do. From an alga sensing light, to controlling cells in a worm, to interrogating circuits in the mammalian brain - the path of discovery was anything but obvious at the beginning, but ultimately led to fascinating science.

Why do scientists study small models like worms, flies and algae?

This brings us to a question students often ask: if we want to understand human disease, why study a worm, or a fly, yeast, zebrafish or an alga?

The answer lies in one of biology's most fascinating features.

Life is extraordinarily diverse, but evolution repeatedly uses and modifies successful biological mechanisms. Organisms separated by hundreds of millions of years of evolution can therefore share fundamental genes, proteins and cellular processes.

Scientists beneficially exploit this conservation by choosing organisms in which a difficult biological question becomes experimentally simpler.

C. elegans, the organism used in that early optogenetics experiment, is a good example.

It is about a millimetre long, transparent and has a rapid life cycle. An adult hermaphrodite has 959 somatic cells and 302 neurons. Individual cells can be identified and followed, genes can be manipulated, and the consequences can often be observed in the entire living animal. Its apparent simplicity has allowed scientists to answer questions that would have been extraordinarily difficult to approach first in humans or even in mammals. Research using C. elegans produced the first complete cell lineage of a multicellular animal, the first complete wiring diagram of an animal nervous system, and the first fully sequenced genome of a multicellular animal.

More importantly, experiments with this little worm contributed to discoveries of fundamental biological mechanisms that turned out to matter far beyond the worm itself.

When small questions become big discoveries

The history of utility of C. elegans in research, is worth reflecting upon.

Studies of how cells develop and disappear in C. elegans helped reveal the genetic control of programmed cell death, or apoptosis. Sydney Brenner, John Sulston and Robert Horvitz received the 2002 Nobel Prize for this body of work. Today, apoptosis is fundamental to our understanding of normal development as well as diseases such as cancer and neurodegeneration.

In 2006, Andrew Fire and Craig Mello received the Nobel Prize for discovering RNA interference (RNAi) through experiments in C. elegans. A surprising observation about double-stranded RNA revealed an entirely new mechanism for switching genes off. RNAi became an indispensable laboratory tool and, eventually, the basis of medicines.

Another beautiful example came from an entirely different organism. A fluorescent protein had originally been discovered in a jellyfish. Martin Chalfie realised that this Green Fluorescent Protein- GFP-might be used as a biological marker and demonstrated the idea in transparent C. elegans. Scientists could now make selected cells or proteins literally glow inside living organisms. GFP became one of the most widely used tools in modern biology, and the work contributed to the 2008 Nobel Prize in Chemistry.

Then, in 2024, Victor Ambros and Gary Ruvkun received the Nobel Prize for the discovery of microRNA. That story, too, began not with human disease but with researchers trying to understand developmental timing in C. elegans. A puzzling observation in a worm eventually revealed an entirely new layer of gene regulation.

There is an important distinction here: the 2026 Nobel is not a C. elegans Nobel. Its scientific foundations span algal photobiology, ion-channel biophysics, molecular engineering and mammalian neuroscience. But the 2005 worm experiment is a fascinating early chapter in the development of the field - and another example of how discoveries move between apparently unrelated organisms.

The unpredictable route from curiosity to medicine

There is a broader lesson in all of this, especially for young people entering science.

Today we understandably ask researchers about impact. What disease will this cure? What product will emerge? Can it be translated? How soon will it benefit society? These are legitimate questions. But if they become the only questions we ask, science loses something essential.

Imagine if these researchers were asked during early days:

What is the medical relevance of knowing how an alga swims towards light? Why spend years tracing every cell in a tiny worm? Why investigate a strange developmental mutant? What possible medical value is there in understanding why a jellyfish glows?

At the time, nobody could have confidently answered: this will lead to optogenetics, apoptosis biology, microRNAs or GFP. That knowledge emerged because scientists first tried to understand nature. Application followed understanding - not always immediately, and rarely along a straight line or following an easy path.

Why this resonates personally?

Having worked with C. elegans for many years, I find this aspect of science particularly rewarding.

In our laboratory, we use the worm to investigate questions related to neurodegeneration, ageing, proteostasis, glial biology, mitochondrial function and the gut-brain axis.

How can events occurring in the gut influence neurons? Can metabolites produced or modified by gut microbes alter neurodegeneration or enhance neuroprotection? Why do some neurons withstand cellular stress better than others? How do mitochondrial function, protein quality control and ageing intersect with neuronal survival? These are large questions. The worm gives us a way of breaking them into experimentally manageable pieces while still studying them in a while organismal condition. One can relatively easily perturb a gene or a metabolic pathway, observe an identified neuron, measure a behavioural consequence and then ask whether the mechanism is conserved in more complex systems.

That does not mean that what happens in a worm automatically happens in humans. Model organisms are starting points for discovering principles, not miniature substitutes for people. That distinction is important - and is also what makes good science both cautious and exciting - ofcourse validations in higher models provide further understanding and research requirements of regulators have to be met before any fundamental research translates to human use.

The pleasure of not knowing where a question may lead

Perhaps the most inspiring aspect of the 2026 Nobel story is not simply that scientists learned to control neurons with light. It is the route by which they got there.

Someone studied an alga because its response to light was biologically interesting. Others worked out how its unusual proteins functioned. Someone then wondered whether those proteins could work in another kind of cell. Researchers tested the idea in different experimental systems. Neuroscientists recognised what this could make possible. Each answer generated another question.

And over time, an observation in a microscopic organism became a technology capable of probing some of the most complex circuitry we know- the human brain. This is how science often advances: not as a straight road from problem to solution, but as a chain of curiosity, observation, experimentation, failure, insight and unexpected connections.

That may be the most important educational message of this year's Nobel Prize.

We certainly need scientists who can translate discoveries into technologies and medicines. But we equally need scientists who remain fascinated enough by nature to ask questions before anyone knows whether the answers will be useful. Because somewhere, perhaps in an alga responding to light, a worm behaving strangely, or another apparently insignificant observation in a laboratory, may lie the beginning of the next major discovery.

In science, the size of the question cannot always be judged by the size of the organism in which we first ask it, hence, never under estimate the power of a small model organism.