Darwin had noticed that over time, living things seemed to change to better adapt to their environment. But he did not understand how these changes were passed from one generation to the next.
By Stephen Buranyi
As strange as it may sound, scientists still don't know the answers to some of the most fundamental questions about how life evolved on Earth. For example, take the case of the eyes. Where exactly do they come from? The usual explanation of how we got these highly complex organs rests on the theory of natural selection.
You may remember the gist from biology lectures at school. If a creature with poor vision happens to produce offspring with slightly better vision, thanks to random mutations, then this slightly better vision gives them a better chance of survival.
The longer they survive, the more chance they have to reproduce, and to pass on the genes that provided them with slightly better eyesight to the next generation. On the other hand, some of their offspring may have better eyesight than their parents, making their reproduction more likely. And so on.
Generation after generation, over very long periods of time, small advantages add up. Eventually, after a few hundred million years, we have creatures that can see as well as humans, cats, or owls. This is the basic story of evolution, as told in many textbooks and bestselling science books.
But according to a growing number of scientists, this theory is too general and therefore incorrect in most cases. First, in the case of the eyes the story starts in the middle, taking for granted the existence of light-sensitive cells, the lens, and the iris, without first explaining where they came from.
And it's not just the eyes. "The first eye, the first arm, the first placenta. How are they created? Explaining them is the basic motivation of evolutionary biology. But we don't have a good answer yet. The classic idea of gradual change has already fallen," says Armin Mozek, biologist at Indiana University, USA.
Of course, there are still some fundamental evolutionary principles that are not questioned by any scientist. So everyone agrees that natural selection plays a role in evolution, as do mutation and chance. But how exactly these processes interact, and whether other forces play a role, remains the subject of intense debate.
“If we can’t explain things with the tools we have now, we have to find new ways,” Yale University biologist Günter Wagner told me. In 2014, eight scientists decided to take on this challenge. They published an article in the journal Nature with the question “Does the Theory of Evolution Need a Revision?”
Their answer was: "Yes, and even urgently." Each of the authors came from scientific disciplines of recent decades, from the study of how organisms change their environment to reduce the normal pressure of natural selection—think beavers building dams—to new research that show that chemical modifications added to DNA during our lifetimes can be passed on to our offspring.
The authors sought a new understanding of evolution that could make room for such discoveries. The name they gave this new framework was the Extended Evolutionary Synthesis (EES). But for many colleagues their proposals were controversial. Behind the current debate on evolution lies a broken dream.
At the beginning of the 20th century, many biologists searched for a unifying theory that would allow their field to merge with physics and chemistry. From today's perspective, it is clear that Darwin's Theory of Evolution—a simple, elegant theory that explains how a single force, or natural selection, shaped the entire development of life on Earth—would play the role of the great unifier.
But at the beginning of the last century, 4 decades after the publication of the book "On the origin of species", and 2 decades after his death, Darwin's ideas were in decline. Scientists had not lost interest in evolution, but many of them found Darwin's account of it insufficient.
A major problem was that it lacked an explanation of inheritance. Darwin had noticed that over time, living things seemed to change to better suit their environment. But he did not understand how these changes passed from one generation to the next. In the early XNUMXth century, the rediscovery of the work of the XNUMXth century priest and father of genetics, Gregor Mendel, began to provide answers. Scientists working in the new field of genetics discovered the rules governing the characteristics of heredity. But instead of confirming Darwin's theory, they complicated it. Breeding seemed to reshuffle genes—the mysterious units that program physical traits—in surprising ways. Think about how the grandfather's red hair is missing from his son, but can reappear in his granddaughter.
An even more disturbing development for Darwinists was the emergence in the 1910s of the "mutationists," a school of geneticists whose leading figure, Thomas Morgan, demonstrated that by breeding millions of fruit flies—and sometimes sprinkling their food with the radioactive element radium – it can produce altered features such as new eye colors or extra limbs.
These were not the small random variations on which Darwin's theory was built, but sudden, dramatic changes. And it turned out that these mutations were hereditary. The mutationists believed they had identified the true creative force of life. Of course, natural selection helped to weed out unsuitable changes.
But she was just a noisy editor of the wonderful poetry of mutation. "Natura non facit saltum", Darwin once wrote: "Nature does not dance". The mutationists prayed for things to change. In the 1920s and 1930s, working separately, the British Ronald Fisher and the American animal husbandman Saul Wright proposed a revised theory of evolution that summarized the scientific advances since Darwin's death but still promised to explain all the mysteries of life with some simple rules.
In 1942, the English biologist Julian Huxley coined the name for this theory: the modern synthesis. Eighty years later, it still provides the basic framework for evolutionary biology, as it is taught to millions of pupils and students each year. By constructing statistical models of animal populations that were based on the laws of genetics and mutation, modern syntheticists showed that over long periods of time, natural selection still worked as Darwin had predicted.
In all of time, mutations were too rare to be important, and the rules of inheritance did not affect the overall power of natural selection. Through a gradual process, advantageous genes were preserved over time, while others that did not confer an advantage disappeared.
But before long, even the modern synthesis would be criticized by scientists within the departments who had helped to formulate this theory. In 1959, the biologist CH Waddington said that the modern synthesis had overlooked valid theories in favor of "drastic simplifications which can lead us to a false picture of how the evolutionary process works".
Then from the late 1960s came some new discoveries, which questioned the foundations of the theory. While modern syntheticists looked at life as if through a telescope, studying the development of large populations over infinite periods of time, molecular biologists saw it through a microscope, focusing on individual molecules.
They discovered that natural selection was not the all-powerful force that many had assumed, and discovered that the molecules in our cells – and thus the gene sequences behind them – were changing at a very high rate.
Biologist Eugene Kunin thinks that people should get used to theories that don't fit together. "In my opinion there is not - there cannot be - a single theory of evolution. There cannot be a single theory for everything. Even physicists don't have a theory for everything," he says.
Taken with abbreviations from "The Guardian" – Bota.al
edition
t7 live
