Among amphibians, the Hydromantes salamander is the champion in tongue firing speed. In less than five milliseconds, she can catch an unfortunate insect in flight - this time includes the work of muscles, cartilage and parts of the skeleton. If you compare this ballistic anatomy with frogs and chameleons, the latter are sloops. David Wake, an evolutionary biologist at the University of California, Berkeley, says: “I have spent about 50 years studying the evolution of salamander languages. This is really interesting, because in general they do not differ in high speed, but nevertheless they can make the fastest movement of those that are available to vertebrates known to me. Throughout their development, evolution has found a more efficient way to ensure successful hunting with language. Their seemingly unique adaptation seems to beindependently developed in three unrelated salamander species. This is an example of convergent evolution, when different individuals independently develop similar biological adaptations under the influence of the same environmental factors. Salamanders are a favorite example that Wake cites when asked the long-standing question of evolutionary biology: If you rewind the tape of evolution, will it repeat itself? Apparently this is what happened in the case of the salamanders; with other organisms this might not have happened.if you rewind the tape of evolution, will it repeat itself? Apparently this is what happened in the case of the salamanders; with other organisms this might not have happened.if you rewind the tape of evolution, will it repeat itself? Apparently this is what happened in the case of the salamanders; with other organisms this might not have happened.
This question is known to be posed for the first time by the recently deceased evolutionary biologist Stephen Jay Gould in 1989 in his book Amazing Life: The Burgess Shales and the Nature of History, which was published in an era when people were still listening to music on cassette tapes. The book told about the fossils found in the Burgess shale, left over from a myriad of strange animals that lived on our planet about 520 million years ago, during the Cambrian period. Almost all animals that exist today have ancestors who lived in the Cambrian, but not all animals from that era have descendants in our era. Many Cambrian individuals became extinct because they were insufficiently suited to the struggle for survival, or because they were in the wrong place at the wrong time when volcanoes erupted, meteorites fell, or other devastating events occurred.
Gould saw the incredible variety of animal remains in Burgess and speculated that our flora and fauna would look different if history had turned the other way. He suggested that chaotic mutations and extinctions of species, which he called "historical accidents," would build on top of each other, moving evolution in one direction or another. According to Gould, the existence of any animal, including humans, is a rare phenomenon, the repetition of which, in the case of "rewind and launch" from the Cambrian period, is unlikely. In his book, Gould often refers to the work on the Burgess fossil by the paleontologist Simon Conway Morris of the University of Cambridge, but the scientist himself strongly disagrees with Gould's point of view.
Conway Morris believes that over time, natural selection forces organisms to undergo a series of adaptations to fill the Earth's limited ecological niches. This leads to the fact that unrelated species consistently converge in body structure. “Animals have to build themselves in accordance with the physical, chemical and biological requirements of this world,” he said. Conway is convinced that such restrictions make it almost inevitable that in the case of "rewinding the tape" evolution would sooner or later lead to the emergence of organisms similar to those that exist in our world. If our monkey ancestors had not developed a brain and the mind attached to it, according to the scientist, another branch like crows or dolphins could occupy the niche in which man is now. But Gould disagrees.
Both scientists recognize that randomness and convergence (independent development until the appearance of similar signs - approx. New why) take place in evolution. Instead, the discussion focuses on how unique or repeatable key adaptations like the human mind are. In the meantime, other biologists have tackled the puzzle and have shown how convergence and randomness affect each other. Understanding the interaction of these forces can help us figure out whether everything living is the result of 7 billion years of coincidences, or whether all of us - humans and salamanders - are part of inevitability, like death or taxes.
Instead of trying to recreate history using fossils, Richard Lenski, an evolutionary biologist at the University of Michigan, decided to observe the phenomena of convergence and chance in real time in the controlled environment of his laboratory. In 1988, he divided the population of Escherichia coli bacteria and placed them in 12 separate reservoirs of liquid culture media, thus allowing them to grow independently of each other. For 26 years now, every few months, he or one of his students has been freezing one batch of bacteria. This frozen germ kit gives Richard the ability to "restart the film" of the E. coli life cycle from any moment he wants by simply defrosting one portion. During the whole process, he can check,how bacteria change - both in terms of genetics and in terms of what can only be seen under a microscope. Lenski explains: "The whole experiment was set up to test how repeatable evolution is."
In 11 Lenski's reservoirs, E. coli grew in size, but bacteria in the twelfth sample split into two independent branches - one with large cells, the other with small ones. Lenski says: “We call them 'big' and 'small'. They have coexisted for 50 thousand generations already”. This has not happened in any other population; hence, we can conclude that an evolutionarily random event occurred. And even 26 years later, no other trial has repeated the appearance of such a branch. Thus, in this situation, chance seems to have prevailed over convergence.
In 2003, there was another accidental episode. The number of rods in one of the reservoirs has increased to such an extent that the culture medium, which is normally transparent, becomes cloudy. At first, Lenski decided that there was a normal contamination of the environment, but as it turned out, E. coli, which normally ate only glucose dissolved in liquid, developed the ability to consume another element contained in the tanks: citrate. After 15 years and 31,500 generations, only one of the colonies was able to process this substance. The number of bacteria in it began to grow 5 times faster than in other colonies.
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This "historical accident" gave Richard and his graduate Zachary Blount the opportunity to test the likelihood of such an event happening again if they "rewound the tape." Blount selected from storage 72 samples of frozen sticks collected at different stages of the experiment from a population that was later able to incorporate citrate into its metabolism. He thawed them and stimulated their reproduction. Soon, 4 out of 72 samples developed the same ability to consume citrate. Interestingly, these mutations only occurred in populations frozen after a 30,500-generation cycle. Genetic analysis showed that not long before this, several genes went through changes that contributed to the emergence of evolution with the metabolism of citrate. In other words, the ability to absorb citrate depended on the occurrence of other mutations that preceded it. It created a forkchanging the possible paths that future generations can take.
Known as the Long Term Evolutionary Experiment, this E. coli project has now crossed 60,000 generations, giving Richard a solid data set from which to draw conclusions about the interaction of chance and convergence in evolution. Subtle changes in bacteria's DNA, making them larger or more capable of reproducing rapidly, have become frequent events in various reservoirs. At the same time, Lenski witnessed "startling" random events in which something completely different from the others took place in one of the populations. But as in the phenomenon of convergence, such transformations were not completely random.
"Not everything is possible," whatever the process, Wake explains: "Organisms develop in the context of inherited characteristics." Animals cannot transmit mutations that are destructive or prevent reproduction. In the case of the Hydromantes salamander, its ancestors had to overcome a significant limitation: in order to obtain their shooting tongues, it was necessary to sacrifice their lungs. This is because part of this mechanism developed from muscles that were used by their predecessors to pump air into the lungs. Today, this once small and weak muscle has become much larger and stronger. It coils like a spring around the cone-shaped bone at the back of the oral cavity, and when the muscle contracts, the bone creates tension, which shoots the tongue along with its bone apparatus from the mouth. Thus, the ancestors of Hydromantes did not just acquire a mutation,which evolved into a "ballistic language". Instead, this adaptation followed a series of changes that first allowed the creature to overcome its lung dependence on oxygen and float to the surface of the water. Each change depended on the previous one.
The chameleons, in turn, retained their lungs. Rather than tinkering with their anatomy, they developed collagen, allowing the tongue to shoot at prey. At first glance, the languages of salamanders and chameleons are an example of convergence, but if you look closely, it becomes clear that this is not so. It takes a chameleon 20 milliseconds to fire, which is a snail's pace compared to the five milliseconds of salamanders. Why did chameleons get such slow languages? Answer: They faced an obstacle in the path of convergent evolution. The chameleon's tongue is fast enough for them to survive, but they lack the "inherited trait structure" to develop the more deadly ballistic anatomy of salamanders. Chameleons have reached an “adaptive peak,” as biologists say.
In experiments with viruses that infect bacteria - bacteriophages - Harvard biologist David Liu also discovered adaptive peaks. These peaks limit the ability of organisms to converge on one optimal structure. They explain why accidents don't happen often.
Liu wanted to know if identical groups of bacteriophages could independently develop the same enzyme if the same evolutionary pressure was applied to them. He accelerated the evolution of proteins in viruses using a system he called PACE.
During the experiment, viruses that failed to produce an enzyme that Liu needed were removed from the experiment. Only those who had reached the goal remained. Some of them turned out the enzyme "better" than others. In this case, they required the enzyme polymerase, which detects a certain DNA sequence and turns it into RNA, and some polymerases recognized the sequence more accurately than others. Like the comparatively slow language of the chameleons, these viruses have developed adaptations that allow them to survive, but prevent them from getting the best polymerase. Some viruses got stuck at a low peak, some climbed higher.
To understand what biologists mean by adaptive peaks, imagine an area whose topography represents high and low levels of reproductive potential. In the case of Liu's bacteriophages, different populations studied the area, acquiring different mutations. Some ended up on small hills, some on mountains the size of Everest. And so they began to climb to the top they got. Having climbed a low mountain, viruses cannot move to another, higher one. To do this, they will first have to go back down, reducing their chances of survival with each step. It is very difficult to do this, because one must not forget about the survival of the fittest. Which mutation will happen before others - which peak will go to the body - this is a historical accident, which convergent evolution can overcome only with great difficulty,if it can at all.
The timing of the appearance of mutations matters. “Early random events that create a difference in the gene pool can significantly affect whether a beneficial mutation can ultimately affect an organism's survival,” explains Liu. "These accidents reduce the repeatability of evolution." In this experiment, randomness overcame convergence. The events that happened prevented recurrence.
One way in which life can overcome the limitations of adaptive peaks was discovered during the study of digital organisms by computer biologists at Michigan State University Chris Adami and Charles Ofria. They created the computer program Avida, in which digital organisms evolve under conditions set by the experimenter. Avidians mutate, randomly acquiring and losing lines of code that allow them to solve mathematical problems, which increases their ability to reproduce.
In one experiment, the Avidians were tasked with obtaining the ability to solve the complex logical problem of "bitwise identity." Only 4 out of 50 digital populations have developed the code needed to perform the operation. All successful populations initially received many mutations (random lines of code) that complicate the solution of mathematical problems and, therefore, reproduction. Paradoxical as it sounds, Ophria found that early bad mutations play a key role in improving fitness in later generations, possibly because they create genetic diversity from which new random mutations can arise.
Does the rarity of any of the sequences of events confirm that the big turns in evolution are unlikely to happen again? Experimentally, this is true, but Conway Morris firmly says no. “It is foolish to think that there are no accidents at all. The only question is time. He believes that with enough time and genomes of mutation, natural selection will lead life to inevitable adaptations that are best suited for the ecological niche of organisms, regardless of the chances that arise. He believes that one day all the E.coli bacteria in Lenski's experiment will begin to absorb citrate and all Liu viruses will climb their Mount Everest. Moreover, these experiments were carried out in very simple and controlled environments, unlike the complex ecosystems to which life outside the laboratory adapts. Hard to say,the influence of the real world would have changed the experiments.
To date, the biggest flaw in all attempts to answer the film of life question is that biologists can draw conclusions from only one biosphere - the Earth. An encounter with an extraterrestrial organism would tell us a lot. Even if alien organisms do not have DNA, they will most likely exhibit similar evolutionary patterns. They will need some material to be passed on to descendants, guiding the development of organisms and changing over time. As Lenski says, "What is true for E. coli is true for microbes throughout the universe."
Therefore, the same interaction between convergence and chance can be observed on other planets. And if extraterrestrial life is experiencing evolutionary pressure from an environment similar to that experienced by earthly life, people of the future may find aliens who have convergedly developed intelligence similar to ours. On the other hand, if random events accumulate, leading life along unique paths, as Gould suggested, extraterrestrial life can be unusually strange.
Gould believed that humans are "an extremely unlikely evolutionary event." As evidence, he pointed out that in the 2.5 billion years of life on Earth, human intelligence appeared only once. He considered the likelihood that another species would develop an intelligence like ours was ghostly small. From the fact that we may be the only intelligent species in the universe, we can draw conclusions that go beyond biology. "Some see this possibility as a reason for depression," Gould wrote in The Wonderful Life. "I have always considered her invigorating, a source of both freedom and, as a consequence, a moral responsibility."
Zach Zorich
The translation was carried out by the project New