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The life on Earth, as far as we have evidence, can exist only in a complex ecosystems. Such systems have always a strong ability of autoregulation, technically said, feedback. If the part of food chain become overpopulated, it brings more opportunities for predator who adjusts its number back. This is a great “invention” of nature, which man should not have over-estimated. A degree of flexibility of a recent ecosystem (and when an evolutionary break begins) is not quite clear. I have entered in medias res - what is the evolution? According to Darwin's hypothesis, life develops by filling up all of empty space with proper type of organisms. Innumerable variations are tried, most perspective replaces the former one - either globally or endemicaly or even both (see the Survival of the fittest). Something like “gradual enhancements” does not exist - the non-perspective streams die, changes are irreversible. As soon as a more suitable level of "innovation" develops (mineralisation, flourishing in plants, thinking in humans etc.), it quickly an explosivelly fills the world and becomes the dominative one. Often it shakes with a recent system so much that we refer to it as the new era. (Paleozoic, Triassic, Paleocene...)

For example, the mineralisation leaded to so-called “Explosion of the fossil records”, the life on all Earth rapidly changed. We speak about the Cambrium, 543 million years ago. Such drastical change happenned -- in a contrast with milliard years of a slow progress before - In a short period of few million years. I must add to express this and to prevent the confusion -- this was not a gradual process, all living creatures suddenly had built the claws, bones, shell etc. -- this was a real revolution.

A catastrofic change from Mesozoic to Tertiary has left just four from twenty prosperous reptile (or dinosaur) families, plus birds and mammals, their successors. (By the way, it is interesting, how viable the insects are -- many “priciples” have been used since Cambrium - they use their simplicity as a main feature to survive.) 

The development of the grasses, which had quickly conquered the world few scores million years ago, offered the space for a new type of animals. Mammals were not in a down cycle of progress, on the contrary they were able to provide a wide base for choosing (the “fitting to specialize” -- variety of different grass-eating cattle / buffalos / antilopes -- and the rest of such ecosystem) - again, it was the instant change. 

As I have mentioned, there was the mammals on top. What might have happened if the grass showed up a few million years sooner, in the golden times of reptiles? Well, the reptiles might have not not survived this, or they might have developed from themselves some specialized grass-eaters. Very likely, they'd been in such case similar to recent herbivores. 

This may sound as a surprising assertion, so I'll try to describe more the process of convergence. The first type - when you compare giant tortoise of Galapagos archipelago with the Aldabra's giant tortoises, if you are not professional, you see not much difference. I remind that they each other live at aprox. opposite spot on globe and they have developed separately, probably from the different ancestors. The Indian star tortoise Geochelone elegans has exactly the same, out of the steppe environment very eccentric shell pattern as the Madagascan radiated tortoiseAsterochelys radiata. All this similarities was formed for the most effective function by the same type of environment. 

The second illustrative type of convergence - Australian Monotremata are a party in exactly same food chain as the South American fauna of the same latitude. Corresponding animal types on both continents look very similar.

The last type - in the same environment and conditions looked the prehistoric pre-reptiles and amphibians exactly the same. In many cases is the only possible method how to determine, if the fossilized remains belonged to an animal belonging among reptiles or mammals is to find a fossilized embryo or egg. Similar situation is in other passing animal groups (fish -> amphibians -> reptiles -> birds, reptiles -> mammals etc.) In completely different types of living creatures often work the same organs developed under the similar pressure of environment. 

Back to evolution - I'd like to correct what is the matter: Beg your pardon, a too much simple idea presented by a very imperfect teachers (or had you better?) in the elementary schools, the idea of some ladder or a tree with a *supreme* human on its top -- is a silly stupid nonsense (by the way, the apes are on a quite non-progressive unimportant branch of the tree ... speaking not about the humans). I could accept an idea of a coral roof with the various endings in the various level ... where the bottom is dead, turned into a stone. To say that “human developed from an ape” is crazy ... Correct is that we became from the similar ancestors, of course.

Our view to the recent species is just the momentary view. Well, we may point to a skeleton and say “yes this species was before this latter which became from the first” -- but simply, if there was a pressure to develop a new form, the older form was abandoned by the innovative and must sooner or later vanish -- or change.

Recent humans themselves are being referred as Homo sapiens sapiens only, while species / subspecies are thought to be extinct (H. s. neanderthalis) or the direct ancestors (H. s. cromagnonis). My personal view -- do not be so straight and strict: For instance in turtles (just around 250 species / subspecies) keep the taxonomy changing every month or so. May we try this experiment -- use a sample of the male humans, do not shave them several days and ... confront them with the “fossil” reconstructions .... by the way very often extrapolated from just one half of some bone ... Have fun! :-)))

I feel human beings as a result of less or more domestic / artificial breeding (same as dogs, cattle etc.) -- not a big difference in the racial aspect here -- so I see this rather as a complex mixture of many types of “man” creating the mankind .... species or not, there is no reason to have this labeled .... 

With a similar scepticism could be interpreted the passing phases / rapid changes of life in past. The scientists - certainly - develop new and new models of the first creature which stepped over the treshold and maight be called the first of all birds ... but ... do they have a single bone? The reason might be, as I have written above, the quickness of the changes. Above this, it is very significant, that the intervals between the massive changes (even called as catastrofical extermination of nearly all former life forms) becoming shorter, as well as the duracy of the periods between the changes themselves.

It is possible, for example, to find the common ancestor of the turtles and the birds, where the mutations come to effect and some of them starts to develop more and more sofisticated shell while others develop the features allowing them to use the floating movement in the air ... (btw. I must notice that this is too often based on the desired construction, not too much on evidence). The conquer of the air itself is immediate, just the same idea of using the available life space.

To say it differently, the success in evolution means to destroy the former order by step up of this ... literaly :) Sometimes the ecosystem survive it and the other forms supply the novelty to maximum funcionality, sometimes is the change total.

A really interesting is the view on the chelonian evolution by relations and successions. First of all, the convergence is not a main concept for everything. We can realize how far the origin of species must be determined -- A simple fact, that prehistoric continents Gondwana and Laurasia divided more than 150 million years ago. In that time from the former ancestors in Gondwana formed the Cryptodira concept in turtles while in Laurasia Pleurodira - sideneck turtles (there ARE exceptions, I will mention them later - those are anyway much younger tribes). Anyway, much sooner than this, at the beginning of perm, there must have prevailed at the „south“ of Pangea (Gondwana) the tribes orinted to develop the Cryptodirae because in trias the Australia was separated - and the sidenecks do not differ in so much signs from their African relatives ... The missing fossil link must be covered by the ice of Antarctis ... so I am safe and must not bring any evidence to this hypothesis :-) which allows me to stay just ironical as well as in the rest of this article.

It might seem – and I found it quite logical at the first sight – that the freshwater turtles, with lower ambient pressure to change, mostly tropical ones, may retain the more primitive formswhile the ones living in the temperate climate kept to develop. I mention for the comparation, that the equatorial Africa has separated from the South America 141 mil. years ago, while the South American Pelomedusidae do resemble the African ones very much. On the other hand, Europe has separated from the North America aprox. 47 mil. years ago (compare the European pond turtle with the Blanding's turtle). The way from the Asia to Northern America was used to migration of the turtles in much closer times (22 mil. ?), when, for example, the box turtles spread up in Asia and North America but not in Europe which was half-covered by the shallow sea that time. A wide distribution of similar concept (I mean here the plastral hinge) in Africa might have been through the "Indian" subcontinent existing at that time ... but it is not too much supported by the fact, that in Africa such feature have some Cryptodira i.e. land tortoises – so it seems that this is the effect of convergence.

I promised the exception - South America. While the gene pool of the connected north continents (America, Europe, Asia) was sufficient for the massive evolution, the progress in the South America was strongly endemical - last time there was the access between the other continents 65 mil. years ago, the reconnection happened 2 mil. years ago. At that time the northern species came to south and exterminated in most cases the less adaptable species. This must have been the mess really...

Smoothly I will change the subject to the tortoises. 200 mil. years ago is the time when Triasochelys dux came, most likely the first true turtle / tortoise. Asia and North America seem to bee the edges of the distribution of tortoises, while the heart of their population lies in Africa, towards to west the South America, eastward India, to north Europe. In Australia they miss entirely, from the mentioned heart the difference is getting bigger. The appearance of African spurred tortoises, leopard tortoises, and American big species (G. chilensis, G. agassizi) is so, that I would say that their ancestors must be more than 141 mil. years old .... South and North American species are so similar, that I agree with the hypothesis stating that less than 2 mil. years ago they spreaded from the same origin. Mediterranean species and Asian ones probably uprisen from the common ancestors of tortoises quite recently, after the reconnection the Africa with Europe -- to develop themselves from their ancestors along with the freshwater turtles looks like a very improbable idea. I may estimate it to the period in the miocen (13-15 mil. years ago), when Europe ceased to be isolated from Asia by the sea -- in that area, which is today characteristic by the distribution of Testudo while souther (India) is widespread G. elegans, relative to south-east African species.

I am usualy a bit self-conscious about the topic of the glacial periods – the tortoises in Europe must be extinct that time, but the reintroduction on just several thousand years seems too short. Nothing is impossible as it looks -- there is a concept about the Testudo kleinmanni to classify them as a very recent species -- just couple thousand years -- diversed and evolved from the common ancestor with Protestudo hermanni.

Table of history (beginnings of periods)

Forming of the Earth? 4 - 5 000 mil.
Archeozoicnon-oxygen mass, zymotic breathing, protozoans3 800 mil.
Proterozoicmulticellular, algae, funghi2 500 mil.
Paleozoicarthropods, briareum, molluscs580 mil.
  (Carbon)vertebrates, fishes, amphibians, conquest of the land345 mil.
  (Perm)reptiles, first turtles (Protestudinata), disaster of amphibians280 mil.
Mesozoicdomination of dinosaurs225 mil.
  (Cretaceous)turtles develop a head-hiding, birds, mammals, beginning of flowers135 mil.
Tertiarythe end of sporozoans, spreading of gymnosperm plants, woody plants and angiosperms, domination of mammals, disaster of reptiles65 mil.
Pleistocenedawning of a human race, glacials alternate quickly2 mil.

Some of used terms

On the contrary to a common view, agression works only within one species (or in a group of the same food specialists) as a symptom of dominance or self-defence.

Little bit generalised - this term means all components of a food chain. High variability, autoregulation coming from itself - works simply in a way which is for a given ecosystem the most balanced one.

See text. This is an “ability” of living mass in developing of very similar look, function of organs, distinctive markings etc. (though developed in really different ways) based on similar lifestyle and environment only.

Survival of the fittest
This is an ability of the fittest within one genus, or within a group of competetive habitat and the food sources. See also a term convergence, which explains a filling of the similar habitats with the similar life forms. See also Agression.

Irreversibility (Doll's theory)
Quotation from the book: Musil - Genesis, developement and extintion of mammals: „... A very nice example of irreversibility of progress show the turtles (Testudinata). They are reptiles, primarily inhabiting the land areas, who have the body protected with shell. During the evolution some groups changed the lifestyle to the life in water, while the shell, which was important for the terrestric lifestyle, was a bit redundant in the water. So slowly this mark was fading off. Na spodní i svrchní straně On the bottom of the shell approached a small hole which kept grow to that extent, that shell nearly disappeared and just small bits remained. Some groups of sea turtles later returned to the terrestric lifestyle and then again, the need of shell appeared -- and it really developed, but not by the enlargement of the rests of the former shell, but in a different way, from the skin. It looks like a funny story, but at the end of Tertiary period some of such groups returned to sea and except of the small remains on the ventral side this secondary shell disapeared too. Today, instead of the bony structure of shell there can be find just the skin with rudiments of two shells over each other. ...“

I'd like to add -- it is far simplier to begin from the start, not only improve the obsolete concept. From this point of view, all chelonia are a great example of very well adapted and what is better, adaptible creatures. However, we must strictly avoid to enlarge a pressure on their extintion (a typical human behaviour) -- though they are still holding on for such a long time.