The Wonders of Life

There’s no question about it: life is immense. Faust’s question about “whatever holds The world together in its inmost folds” remained unanswered for millennia. It was only with the Enlightenment that humans began to subdue the world intellectually, piece by piece. Based on everything we know today, the two decisive factors in the origin and evolution of life were chance and time. Given enough of both, amino acids can eventually form proteins, and ultimately, cells. Cells can then eventually come together to form multicellular organisms, develop so-called gonads specialized for reproduction, and thus pass on their genetic material. From generation to generation, the genetic material is constantly changing. Early on, numerous “blueprints” emerged that give the classes and orders of animals their characteristic appearance. Some aspects of these blueprints have proven to be fundamental – for example, most vertebrates have limbs. Yet some species have legs, some have fins, some have wings, and a few have neither.

A nervous system is virtually always included in the blueprint. This is because every new generation of life faces the same problem: to survive, it must adapt as best as possible to the prevailing conditions. In ancient times, for example, to the cellular toxin oxygen; to an overwhelming number of predators; or, conversely, to a lack of prey.
Nervous systems help organisms cope with life’s uncertainties and ensure the survival of the next generation.

This Adaptation led to astonishing solutions: Single-celled organisms with flagella can move away from sources of danger and toward nutrients. Organisms that develop sensory receptors for light, temperature, or sound waves gain vital information about their environment from these stimuli. Those that expand their motor capabilities can move faster and/or farther. 

The need to link a stimulus with the appropriate response gave rise to the first nervous systems: simple neural networks, such as those of the freshwater polyp Hydra or jellyfish, in which the sensory system communicates directly with the motor system. Of course, this is not enough for higher organisms. And since life usually feeds on other life, a kind of arms race developed over millions of years: Insects that once felt safe in the darkness of night eventually found themselves confronted by nocturnal predators such as bats or owls. Completely different sensory systems – echolocation on one side, incredible hearing on the other – enabled these two species to reliably forage within their respective nocturnal evolutionary niches. But the prey also develops its own tricks – such as the moon moth (Actias luna), which confuses bats with its elegantly elongated wing shape. As mentioned: an evolutionary arms race.

Specialization in specific foods or habitats aids survival. Adaptation to these ecological niches has led to fascinating evolutionary solutions. Let’s stick with owls: Like us and other terrestrial vertebrates, they have an eardrum, Ossicles in the middle ear, and Hair cells in the inner Ear that respond to different frequencies (though the ability to process higher frequencies has only been part of their repertoire for the past 50 million years). An Auditory nerve carries this information to the brain, where it is processed – that’s it.

That sounds simple, but it’s full of subtleties. In barn owls, for example, the ears are positioned asymmetrically, which makes it easier to locate the sound source. Their auditory system is accordingly designed to process the incoming signals optimally – the inferior Nucleus in the owl’s Midbrain contains location-specific neurons, each of which responds to specific areas in space, creating an acoustic map of the surroundings. This and several other unique features of the owl’s nervous system – such as the integration of time difference and intensity – make the barn owl a highly dangerous nocturnal hunter: it can hear the movement of a mouse even beneath a 50-centimeter-thick blanket of snow.

The South American green knifefish Eigenmannia virescens has developed entirely different abilities driven by completely different needs: It senses its environment via a special organ that produces electric fields. Objects and organisms in its surroundings – such as sand, plants, or even other fish – disrupt these fields, which the knifefish in turn perceives through additional organs in its skin. This system would, of course, become disrupted as soon as two fish encounter each other that are “transmitting” on the same frequency. But even that can be resolved: When they meet, both fish simply change their frequency – one up, the other down. Thanks to this “jamming avoidance response,” the fish are once again able to recognize their own frequency. Of course, this is also made possible by a very specific neural architecture. 

Neuroethology investigates the functional blueprint of such highly specialized nervous systems – it looks directly into the cards of evolution. Today, experts in the field can already assess quite reliably, based on the wiring of an auditory system, how the animal in question hears.

The step from neuroethology to basic research is a small one. For what really happens at the level of individual cells? Neuroscientists such as Eve Marder and Eric Kandel have studied this in great detail in lobsters and the marine snail Aplysia, respectively. Thanks to Aplysia, we know, for example, exactly which Neuron is responsible for triggering the gill retraction reflex in response to a fear-inducing stimulus. It is the same cell in every animal. But that was only the beginning of the story of Kandel and Aplysia. Ultimately, with the help of the sea slug, Kandel uncovered fundamental molecular mechanisms of learning – mechanisms that also operate in mammals. 

To Eve Marder we owe our understanding of just how complex even tiny networks operate. Marder studies the digestive system of crabs, which consists of only 26 neurons. A tiny network in a very simple animal, and yet its output signals respond with astonishing flexibility to input signals and can even change accordingly. 

26 neurons – that really isn’t much. And yet this tiny circuit has been a popular model system in research for decades – and even today continues to surprise large research teams with ever-new insights. 

This type of basic research may not seem particularly significant to some. How relevant can a snail’s gill retraction reflex or a crustacean’s digestive system really be to humans? But as we use increasingly sophisticated methods to examine the mechanisms of various nervous systems with ever-greater precision, an increasingly detailed overall picture emerges: nerve cells organize themselves into networks. They communicate via electrical impulses and chemical messengers. Released by the “sender,” these messengers are detected and interpreted by the “receiver” through receptors and ion channels in the cell membrane. Cascades of downstream signaling molecules on their way to the cell Nucleus initiate Gene expression there and thus the production of proteins. These proteins enable Plasticity – they make these networks flexible. This alters the communication between the involved cells and, ultimately, behavior. This is true for humans as well.

This overall picture is a mechanistic one and no longer sounds wonderful at all: Are we humans, as the crown of creation – which we so readily consider ourselves to be – merely the sum of our nerve cells’ activity?

Let’s look at a very practical example by simply taking a few steps forward. As soon as we do so, our forward movement shifts the image of the environment past our eyes and toward the back. This so-called “optical flow” must be compensated for by the nervous system; otherwise, we’d lose our bearings.

Many living organisms face this complex problem, and the fruit fly solves it with just three cells per brain Hemisphere: These “HS cells” respond to the preferred direction of a light stimulus with increased activity and to the opposite direction with decreased activity. This, in turn, has a direct influence on the running speed of the legs. Things get mechanistic as soon as you reverse this process, as Alex Mauss (then) at the (now) 
 Max Planck Institute for Biological Intelligence has done: By influencing the activity of the HS cells, he can manipulate the running behavior of the tiny fly. In other words, Mauss alters behavior by influencing the activity of specific neurons.

Now, the fruit fly has about 100,000 neurons, and humans have about 86 billion. We are obviously much more complex, and accordingly, brains are often described in terms of the most advanced technology available at the time. The philosophers of the Enlightenment liked to think in terms of the mechanical processes of water pumps and the gears of watchmakers. In the 1970s, it was computers. Today, it’s complex networks. 

Complexity is a complicated concept – it shows just how little we actually know. For wherever we’ve discovered a new mechanism in recent decades, it has always presented us with new questions and often opened up an entirely new world. Tiny discoveries suddenly lead to entirely new fields of brain research, and we uncover a complexity that no one could have imagined before. The best-known example of this is probably artificial Intelligence: In the 1960s, computers were on the verge of achieving it. At least, that’s what computer scientists at the time thought – though they weren’t overestimating future computers, but rather underestimating intelligence and consciousness. 

This happens time and again; most recently, for example, with a 2020 discovery regarding Gene expression. We already know a great deal about this: A section of DNA is activated and opens up to the proteins of the transcriptional machinery. A specific region – that is, the blueprint for a specific protein, since DNA provides nothing else – is transcribed and transported via so-called “messenger RNA” to the Protein synthesis factories outside the cell nucleus, where the protein is produced. This is how it works for every cell. And that’s what every textbook says.

Then came the discovery in question. It concerns invertebrate cephalopods – octopuses and squids. They are apparently able to modify the messenger RNA on its way from the cell Nucleus to the site of production. At the very least, modified proteins are found in their axons that are adapted to the needs of these notoriously intelligent animals. Or, to put it another way, the neurons of cephalopods generate “mutations” by correcting the genetic code – mutations that do not affect the genetic material in the cell nucleus and thus do not alter the organism of the next generation. We’ve only just begun to marvel at the technical and experimental possibilities offered by CRISPR/Cas technology, and now the octopus has outsmarted us.

Life – and especially the nervous system – is far more wondrous than we ever imagined. For despite all the mechanics involved, even single signals can alter the entire system – and thus signal processing. Today’s “cogs” are highly dynamic processes. 

As fascinating as these findings are – basic research often has an image problem: Who cares which gears mesh with which others when a vaccine is needed right now? Applied research seems much more profitable than the aimless generation of fundamental knowledge. But that overlooks the timeline: What seems useless today may be highly relevant tomorrow. What basic researchers discovered about Oligodendrocytes decades ago now forms the crucial foundation for treatments for Multiple sclerosis The more we learn about the mechanisms of cellular metabolism following damage, the better we can treat stroke patients. This relevance becomes apparent in the interplay with applied science. Its successes are made possible – sometimes decades later – by basic research.