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WHAT DID THE FIRST VERTEBRATE LOOK LIKE THAT ENTERED LAND ?
THE STANDPOINT OF INITIAL BIPEDALISM THEORY
Première publication : septembre 1989
Mise en ligne : 20 juin 2003
par François de Sarre
The earliest known vertebrates are the Armoured Fishes of the Silurian. They have had hard skeletons and carapaces, and that is why they were capable of fossilization when they died in seawater-pools or lagoons, under conditions favourable for their preservation.
Incomplete and fragmentary as it is, the geological record only gives indications about ancient faunae. This does not represent the actual links in the evolutionary chain, nor does it demonstrate any evidence of chronological order ( fish, then amphibians, reptiles, birds and mammals ) in which the series of the vertebrates should be listed.
Pisces, indeed, appear to be highly specialized water animals which proceeded from more primitive vertebrates that preserved less evolved features, such as 4 legs and functional lung, that transformed in fish into fins and natatory bladder... The remarkable thing about Crossoptérygians, and also Dipnoi ( lungfish ), is that they preserved choanae or internal nostrils, and lungs for air-breathing [ gills being posteriorly added ].
According to a classical view, the land vertebrates which flourished in Carboniferan times were clumsily built amphibians. However, even though fossil records have not been discovered as yet, there lived logically at the same time with several more completely terrestrial lineages which sprung from some branch of the homonculian stem.
Fossilized mammals are known from early Triassic layers as small creatures said to have originated from reptiles. However, only the mammalian level of structure should have reached such perfection and adaptability to produce set-types which specialized in various ways and at different times during early geological periods. It means, we must believe the contrary, i.e. that reptiles once evolved from mammalian creatures. We can call this process ( an adaptive simplification of body structures ) : reptilization. This entails the fashioning of various reptilian and reptile-like lineages form originel mammals ; and the evolution may develop further to amphibians or to completely aquatic forms, such as the fish.
I’d like briefly to present here my individual concepts about the morphological appearance of the first vertebrate that was intended to conquer the land.
In this article we will not evoke problems as to demonstrate whether a fish looking like a crossopterygian could really have walked on its paired fins [in which the bones have remained comparable to those of the tetrapod fore and hind limbs, or we will not discuss if such fins should necessarily disappear, as the fish was winding about on land [ like a snake ] by undulating the whole body in a horizontal movement. Let me only stress here the fact that such a creature entering land must have had a real motivation to move forward onto a new and hostile environment !
A small-brained, also physically unadapted, crossopterygian fish would be indeed not capable to undertake such an adventure.
This biological fact seems to have been forgotten, perhaps because observers have adopted the habit of imagining here something like a tadpole that leaves its native pool, before changing into a land-living and air-breathing frog !
The first land vertebrate really had -without any assistance of heredity- to adapt itself to a strange new world, to avoid the drying of its organism, to endure and support the daily changes in temperature, to carry a body that became heavier on land... Food searching was also a problem ! It was necessary to improvise and to make do with what was possible to find on land : algae, perhaps a few animals with shells, or arthropods.
During the Precambrian time, the archaic vertebrate already made the decisive step towards land-conquest, leaving the ocean where its ancestor have lived. Our present human endeavour and trend to travel through countries and to reach new unexplored places is still a continuation of this fundamental trend of archaic pre-hominids !
The Homonculus in its archepagoge [ full aquatic ] stage has just developed its brain, its osseous skull and its limbs. The spinal column, quite upright, was indeed ossified before the pre-hominid left the water ! Dessication and thermic regulation of the whole body were the chief problems for a creature that was trying to make the ’big step’ out of the ocean... On land, the differences in temperature are large and sudden.
It was very essential, too, to avoid the dessication of the body. The skin of the first land vertebrate had to be kept supple, but also covered with an outer horny layer that would restrict the loss of water, and owning an isolating hairy coat ! The sweat glands in the skin did intervene in case of too much heat, producing sweat which evaporated and cooled the organism. The sebaceous glands served to lubricate the skin and the hairs.
This is a self-regulatory system, able to maintain a stable body temperature, despite external variations. Such control permitted an adaptation to life on land after past life in the ocean.
The development of teeth which allow the Homonculus to chew and to digest its food more quickly is connected with the acquisition of homeothermy, which also requires a great need of energy. The apparition of isolating hairs is indeed connected with the demand for thermoregulation. All these characters were present before the end of the aquatic stage !
The globular form of the human skull, also a primitive ( plesiomorphic ) feature, represents the final evolution of the sea-living Homonculus’s floating and sustenance organ : such a round configuration could only develop naturally in water !
This ocean-dwelling pre-hominid then started to evolve into the first land-living ’true’ vertebrate. This is what I refer to as ’Homonculus in his phytophore stage’, or ’archaic man’. As we have also emphasized, this creature had an inborn tendency to explore, and he possessed an effective nervous system [ the big brain developed from the marine floating organ ]. The Homonculus already had the required physical pre-adaptations, such as : bipedal gait and orthograde body-position, air-breathing, thermoregulation, and digestive organs that allowed him to eat the terrestrial plants and the small animals that were living all around.
Hypothetical reconstruction of the Homonculus
at his phytophore stage,
as the first vertebrate that ever entered land
Bipedalism certainly preadapted the early hominids to a life on land. The upright position was already acquired before leaving the ocean lagoons. Further modifications of the post-cranial skeleton, such as the configuration of the plantigrade foot, resulted from the constraints that were imposed by the environment, and by an active awareness of these new terrestrial habits.
It was, indeed, in remote times, the completion of the human form, and the conception of the original type of placentary mammals. The phytophore Homonculus presented the important new characteristic of conceiving and bringing into world living young [ offsprings ]. Before the birth, the childs grows and develops in his mother’s uterus. The placenta is the organ that feeds and discharges it from wastage [ through the umbilical cord ], while the amnios is the cavity filled with water where the ftus floats.
A derived feature is that of the mammals without placenta ( Marsupialia ), and of other vertebrates ! There may be a ’more evolved’ feature in the egg-laying of birds, reptiles and also of some mammals ( Monotremata )... where the embryo [ which still needs to develop in water ] is surrounded with the same amnios. This organ then becomes superfluous when the level of organization of amphibian and fish is reached ! They indeed constantly live in water, or use water for their larvae [ which exhibit aquatic specialization : this allows the species to spread through lakes and rivers ! ]. The eggs of fishes and of amphibians are consequently without calcarious shell, and don’t develop the amnios or allantois of ’higher vertebrates’.
Now, thanks to his viviparity, the Homonculus on land should have assured a good rate of reproduction. Above all, this disposition permitted a complete growth of the big brain in the uterus, and preserved the globular form of the human ’primitive’ skull. Viviparity also allowed a definitive independence of all aquatic contingencies.
Just after this hard step of leaving the water, the terrestrial Homonculus was ready to colonize this new milieu that became open to it. By pursuing their structural evolution ( specialization ) beyond the achievement point of the human form, diverse mammalian creatures could arise... a part of them became quadrupeds by adaptation, but all belonged originally to our own bipedal line of ascent !
These animals established on dry land and in many other ( for exemple, semi-aquatic ) environments, or also reintroduced the ocean. In this way, fishes ( Pisces ) themselves have returned secondarly to life in water, just like some reptiles or mammals did. In such a hypothesis, today ( and fossil ) groups of fishes evolved several times from different ’amphibian’ lineages, which developed themselves from reptiloid quadrupeds.
From his remote prehuman ancestors, man has inherited the ability to stand erect on his hind legs ( initial bipedalism ). The forelimbs, relieved from any locomotory function, were set free - since the beginning - to serve the large brain in providing for its needs.
As a matter of fact, man has remained morphologically and anatomically the same through out the course of geologic ages. Among living - and fossil - vertebrates, he is the least removed from the ancient prototype !
Man preserved the original orthograde body position, a head resting without musculature support on the vertebral column, a big brain and the round-shaped skull, the hands in primitive form and the plantigrade foot.
Many features ( aptitude of perfect bipedal gait ! ) have been lost by today apes, and already by australopithecines and other fossil-known hominoids. The chin, also, kept in man its original characteristics, while it has disappeared in other primates [ included ’Homo erectus’ and similar forms ] because of the strong development of the jaws, the lengthening of the skull, and the specialization of teeth. These various groups of homin(o)ids have followed their own evolution parallel to Homo sapiens’ progression, and at the same time branching out in different lineages. Such a process is called : dehumanization.
Another evolved feature in hominoids may be the development of animal hairs that are added to original human hairy coat. It’s a relevant characteristic of dehumanized beings, demonstrated for instance in Dr. Heuvelmans’ Homo pongoides [ see the illustration to the article of Michel Raynal in the same issue of Bipedia ] and by living apes.
The unborn chimpanzee, indeed, has hair on his head like a human, but little hair on the body. Then, the ape-baby develops his specific hairs. The apparent ’hairlessness’ of man is not due to an absence of hairs, but it is due to the fact that these hairs remain normally small and thin. As a matter of fact, man is not a ’naked ape’... and the apes’ hairs are the result of a more advanced evolution than man never followed ! The strong development of the teeth, jaws, chewing muscles, and mouth of non-human mammals is indeed in inverse proportion to the brain development. At a certain point of its embryogenesis, the animal embryo carries on developing after the point where the human embryo was complete... and the formation of a large brain is countered as dentition and jaws become more powerful !
As a matter of fact, only the human lineage ( Homo sapiens ) preserved a primordial brain disposition and a globular skull [ which were acquired before the passage from an aquatic to a terrestrial existence ]. This indeed carries the problem of anthropogenesis back to a very early stage in the evolution of the vertebrate line...
François de Sarre