#头条创作挑战赛#
It is precisely because our ancestors guarded the five fingers that we counterattacked and became primates.
When you accidentally see your five fingers, you may not think that the genes that determine your five fingers come from the ancient Carboniferous period, 350 million years ago.
If your finger is a separate species, it is a typical living fossil.
419 million years ago, when Guiyu oneiros roamed freely in the water, they probably could not have imagined that one of their descendants would develop limbs and phalanges, conquer continents they had never set foot on, and create one evolutionary legend after another.
The Phantasmal Ghost Fish is one of the first bony fishes, and is also located at the base (bottom) of the Flesh-Fin Fish Evolutionary Tree.
Our upper and lower limbs are derived from their pectoral and pelvic fins.
It can be seen that the main structure of the fin is the fin bar.
Many people may take it for granted that:
The human finger is supposed to have evolved from the fins of a fish, or a completely developed structure.
In fact, early biologists thought the same way, but they were slapped in the face by later research [2] [3] [4].
The membranous bone that makes up the fin bars is essentially a connective tissue that is not homologous to the phalangeal bones of intrachondral osteogenesis.
Therefore, the fingers cannot be developed from the fins, but from the intrachochondral osteogenesis at the base of the fins. That is, the small part of the connection between the fins and the shoulder straps/waistband, which does look very inconspicuous. Looking at 400 million years ago, who would have thought that such a small part could evolve to a full 50% of the entire body weight in the future?
Whether or not intrachondral osteogenesis develops fingers/toes is controlled by the Hox13 gene family.
- The Hox13 gene is not a single gene, but multiple genes distributed on different chromosomes.
A 2016 study published in Nature found that if the genes associated with zebrafish (HOXA13A, HOXA13B, HOXD13A) were knocked out, their fins developed very short. Even as the fin bar is shortened, more intrachogenetic bone appears at the base, which is the same type of bone that makes up the fingers. Conversely, if the associated genes (Hoxa13, Hoxd13) are knocked out in mice, they are unable to develop hand structures.
From this experiment, we can speculate that in the evolution of flesh-finned fish, it is likely that the transition from fin development to phalanges is guided by turning off/turning on the related Hox13 gene. To this day, the occasional occurrence of polydactyly in humans is associated with mutations in the HOX13-related gene [5].
Evolution itself is a tinkering, adhering to the principle of maximum utilization.
419 million years ago, after 30 million years, a small bit of intracartilaginous osteogenic matrix at the base of the fin of the dream ghost fish gradually developed a large class of fleshy finfish with limb bones and carpal bones, represented by the true palm-finned fish [6] (Eusthenopteron) 390 million years ago, and they have begun to march towards land.
However, they ended up failing miserably.
In addition to the fact that their limbs are not strong enough, one of the main reasons for their failure is that they have not evolved phalanges.
Another 10 million years later, 380 million years ago, the Panderichthys [7] learned the lessons of failure and developed a highly ossified humerus, ulna, and radius, especially the humerus, which was longer than any other fish of the same period. Coupled with having strong shoulder blades and pelvic girdles, they can crawl long enough distances in the shallows.
After crawling on the shallows, they finally developed phalanges after crawling in the shallows.
Although in the beginning, the phalanges were more like scattered bones, the larger area supported by the phalanges not only makes them more powerful to crawl in the shallows, but also allows them to be connected by webbing while maintaining excellent swimming ability.
However, the most taboo in biological evolution is the intermediate ecological niche, which is not high or low, and Pan's fish eventually dies with regrets. However, one of their closer relatives became more and more courageous, constantly reshaping the phalanges, successively developing Tiktaalik (Tiktaalik), Hope Newt (Elpistostege), and finally evolved the typical Phalanges (Acanthostega), Ichthyostega, and Tulerpeton (Tulerpeton) [8] [9] .
Spinous newts have 8 heel phalanges/phalanges, but the middle six are dominant. The fish newt is missing one, which is 7 heel phalanges/phalanges. By the time the newt arrived, there were only six phalanges left.
348 million years ago, a representative organism, Pederpes, appeared [10].
The hind toe has 5 heels and the front toe has 6 heels, and it is considered to be a functionally five-toed animal.
Since then, early tetrapods have gradually developed from a polydactyly to a typical five-fingered type.
However, the five-fingered shape of early tetrapods may not be of single origin, but of multiple origin.
All of today's tetrapods have developed from one of the five-fingered shapes.
All other finger shapes, all of which became extinct shortly after the quadrupeds landed.
In the water environment, the fish advances, mainly facing the resistance of the water, and it is enough to optimize the hydrodynamic structure of the body. But after landing, their bodies and limbs must defy gravity like never before. As a result, the limbs, metacarpal bones, and phalanges are all subject to new adaptations to the stresses generated by land crawling.
Under gravity, their carpal bones (forelimbs) and tarsal bones (hind limbs) are axilly folded forward. The weight of the body is all applied to the carpal and tarsal bones, causing the bones to realign.
The indentation of the two ossicles inside and outside the palm reduces the attachment area of the phalanges, which eventually leads to the degeneration of the marginal phalanges/phalanges, and the number of fingers naturally decreases.
In addition to the reduced first-evolution pressure on the fingers and the need for strength support from the wrist, fewer phalanges can also avoid unnecessary competition between too many fingers, allowing the muscles attached to the palm to be more concentrated, making the hands more flexible and powerful. Therefore, soon after the quadrupeds landed, they gradually mastered the ability to grasp and climb, so that they could quickly radiate and develop into the complex ecological niche of all walks of life, all over the mountains, rivers and lakes.
You may wonder, then why didn't the fingers decrease to less than 4, but remained at the number of 5?
The main reason is that most of the early quadrupeds were amphibians, and they also had a need to swim. Obviously, 5 fingers can support a larger area than fewer fingers, which is good for swimming. In addition, their bones are less strong, and the phalanges that support the body should not be too few.
Although the five fingers are a relatively primitive feature of tetrapods, they have the extraordinary potential to develop in all directions.
The beauty of biological evolution is that primitiveness does not equal failure. The only criterion for failure is to be completely eradicated from the earth's ecology by nature.
With the full-scale journey of vertebrates to the mainland, the primitive five fingers/toes will eventually undergo new and amazing changes with the rise and fall of dynasties.
Terrestrial vertebrates are mainly divided into plantar rows, toe rows, and hoof rows.
The earliest tetrapods were all plantar and did not change their gait dramatically for at least 100 million years. The main reason is that they are basically cold-blooded animals with limited metabolic capacity, and they either can't run fast enough or can't run continuously for long periods of time.
At the end of the Permian period, 250 million years ago, with the emergence of mesothermians and even homeotherms, the activity capacity of animals increased. Not only will the slow herbivores be eaten, but the slow predators will also starve to death.
Those who have experience in running should know that when running at high speed, you can run faster by lifting your heels and gripping your toes on the ground. As the evolution of speed became more and more involuted, the animals that could not maintain a high-speed gait became extinct, and the gait that survived was completely changed and became a complete toe-toed animal. In the beginning, all of the toeed animals had five fingers/toes.
However, the more fingers/toes there are, the more distributed the power and the more visible the running speed.
As a result, as the demand for speed increases, the fingers/toes tend to degenerate. However, for animals with the need to grasp and climb, there is a reverse evolutionary pressure to counteract the loss of toes.
For today's cats, for example, the front paws also have a need to grasp and climb, so the five claws are retained. The hind limbs have a reduced grip demand and are more inclined to run, so they have only four toes.
However, the first animals to show toe loss were not mammals, but dinosaurs.
After the mass extinction event at the end of the Permian, with the decline of the Zygosaurus, the main saurs rose. Since the late Triassic period, 230 million years ago, dinosaurs ruled the earth for more than 100 million years, and their internal turmoil and dynastic changes occurred, during which different branches underwent asynchronous changes. With the exception of the primitive differentiated taxa that firmly occupies the giant niche - sauropods are five-toeded, later taxa have more or less experienced a high degree of evolutionary competition, and most dinosaurs have degenerated their toes and become four-toed animals, some have become three-toed animals, and some even have only two toes, represented by dinosaurs.
Of course, due to the greatly reduced demand for the forelimbs of the middle and late dinosaurs, it was seriously degenerated, and even a one-fingered one-fingered river-clawed dragon appeared.
The change in the fingers/toes reflects the competitive pressure of a species for survival. Their different forms of fingers/toes reflect their former glory. Today's birds still have different toe shapes, which are also closely related to the needs of different habitats such as running and grasping.
Our ancestors, the Zygosaurus branch, experienced the mass extinction at the end of the Permian, the great drought of the Triassic, and eventually only a few small animals evolved into mammals. In order to avoid the dinosaurs, they burrow into the jungle and become nocturnal animals, only to win a glimmer of life.
The long-term stability of ecological niches has also made their evolution extremely slow.
The earliest mammals appeared 225 million years ago, represented by the Cryptokingmon, almost at the same time as the dinosaurs. However, it took nearly 60 million years to develop the earliest viviparous eumammal, the Chinese Jurass, 160 million years ago. In the next 100 million years, the true beast did not change much in size or ecological niche.
In the nocturnal jungle niche, our ancestors not only had the need to climb, dig holes, but also the need to catch insects and pick fruits, all of which have always maintained the primitive characteristics of our hands - the five-fingered state.
65 million years ago, after the extinction of the dinosaurs, the entire continent was once again left with a huge niche vacuum.
Different mammals adapted to different ecological niches, and once again there was a special evolutionary competition, and the limbs changed dramatically again. In particular, the ungulate is the most significant, not only evolved into today's artiodactyls and odd hoofs, but even simply went to the sea again and evolved cetaceans, completely degenerating hind limbs.
However, our ancestral primates, no matter how many vicissitudes it has endured, how many ups and downs, after all, did not choose to degenerate the five fingers, but climbed to the treetops, constantly strengthening the grasp ability and flexibility of the fingers, and then went out of the jungle to constantly temper our hands. It became the most aggressive organ on the planet, and ultimately created its own glory.
Of course, that's the next story. In a sense, it was our ancestors who held on to the original five-finger feature for 350 million years that gave us the opportunity to create human dynasties today.
reference
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- ^Kherdjemil, Yacine, et al. "Evolution of Hoxa11 regulation in vertebrates is linked to the pentadactyl state." Nature 539.7627 (2016): 89-92.
- ^Nakamura, Tetsuya, et al. "Digits and fin rays share common developmental histories." Nature537.7619 (2016): 225-228.
- ^Woltering, Joost M., Michaela Holzem, and Axel Meyer. "Lissamphibian limbs and the origins of tetrapod hox domains." Developmental biology 456.2 (2019): 138-144.
- ^Malik, S., and K‐H. Grzeschik. "Synpolydactyly: clinical and molecular advances." Clinical genetics 73.2 (2008): 113-120.
- ^Laurin, Michel, et al. "A microanatomical and histological study of the paired fin skeleton of the Devonian sarcopterygian Eusthenopteron foordi." Journal of Paleontology 81.1 (2007): 143-153.
- ^Boisvert, Catherine A. "The pelvic fin and girdle of Panderichthys and the origin of tetrapod locomotion." Nature 438.7071 (2005): 1145-1147.
- ^LEBEDEV, OLEG A., and MICHAEL I. COATES. "The postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev." Zoological Journal of the Linnean Society 114.3 (1995): 307-348.
- ^Ahlberg, Per Erik, and Jennifer A. Clack. "Lower jaws, lower tetrapods–a review based on the Devonian genus Acanthostega." Earth and Environmental Science Transactions of The Royal Society of Edinburgh 89.1 (1998): 11-46.
- ^Clack, J. A., and S. M. Finney. "Pederpes finneyae, an articulated tetrapod from the Tournaisian of Western Scotland." Journal of Systematic Palaeontology 2.4 (2005): 311-346.