Another five “evolutionary” old news

Vernanimalcula — the first of us Charles Darwin considered the rapid appearance of most animal phyla at the beginning of the Cambrian period (about 570 million years ago) one of the circumstances unaccounted for by his theory. Today the "Cambrian explosion" seems no less astonishing. The modern fauna numbers 30–35 phyla of multicellular animals; but in the Cambrian, probably, about 100 appeared! After that no new phyla arose, while many old ones disappeared. Most animal phyla are characterized by bilateral symmetry. Their anterior and posterior ends, as well as the dorsal and ventral sides, are arranged differently, whereas the right and left sides of the body are mirror images of each other. To set such polarity in the course of development is no simple task. In all such animals (from flatworms to humans) the determination of body polarity is connected with the action of one and the same group of switch-genes. So, bilateral symmetry was "invented" once! Until the mechanism of developing a complex body plan was optimized, various variants of it arose — the animal phyla. Then the development of most phyla stabilized, but to our day, after harsh trials, only the best of the best have survived. Several decades ago, in Precambrian (Vendian) deposits, diverse organisms were described, sometimes reaching tens of centimetres. Their first finds were made in the Ediacara locality in Australia; there are many of them on the White Sea in Russia too. At first people wanted to see our ancestors in these organisms. Unfortunately, they are hard even to consider animals. Their features: the absence of mouth openings; glide-reflection symmetry (as in a lightning bolt); growth without changing proportions; feeding thanks to the bacteria inhabiting their bodies. At the Vendian–Cambrian boundary they are replaced by animals of our evolutionary group. These are bilaterally symmetrical, change proportions as they grow, consist of diverse organs and tissues, feed through a mouth and, most importantly, are extraordinarily diverse. But where did they come from? And recently Chinese and American palaeontologists found small (0.2 mm) Vendian bilaterally symmetrical animals that lived 580–600 million years ago. A reconstruction of such a creature (named Vernanimalcula — "spring little animal") and a thin section of one of its fossils are shown in the figure. It had a mouth, internal organs and a body cavity! Thus, the Cambrian animals did not arise out of nothing — it is just that their ancestors were small and insignificant. When the chemistry of the water began to favour the development of organisms with hard coverings, the amount of oxygen dissolved in the water increased, and the dominants of the Vendian period disappeared, the relatives of Vernanimalcula began to experiment with the body plan. We are the result of one of those experiments.

About our smaller brethren Before rising to the stars, it is useful to take a good look beneath one's feet. Now, when the count of known stars with planetary systems reaches into the dozens, we must understand how to recognize those on which life exists. It is more or less clear how one should look for a civilization that sends radio signals in search of like-minded beings (or at least broadcasts pop music). However, the age of the Earth is measured in billions of years, while radio stations are little more than a century old from their birth, and it is unknown how long they will last. Four-fifths of the history of life on Earth fell during the dominance of prokaryotic (bacterial) ecosystems. Bacteria are inferior to us in command of radio technology, but greatly surpass us in the diversity of the biochemical reactions they perform. The results of such reactions must be reflected in the composition of the atmospheres of inhabited planets. Fortunately, the composition of gases in planetary envelopes is amenable to remote study (on the basis of analysing absorption and emission spectra). For example, although humanity has sent many research craft to Mars and obtained samples of the Red Planet's matter (in the form of Martian meteorites), to this day the main arguments in the dispute about the existence of life on Mars are connected precisely with the detailed study of the atmosphere's composition. Unfortunately, specialists in extraterrestrial life do not really know what exactly they should be looking for. The chemistry of the early biosphere is clearly insufficiently studied. Seeking to fill this gap in knowledge, the American aerospace agency NASA began studying modern bacterial ecosystems in the Mexican Chihuahua desert. The difficulty of studying ancient ecosystems is that in our time practically none of them remain. After bacteria changed the character of the Earth's atmosphere from reducing to oxidizing and accumulated enough organic matter, the time of the eukaryotes (plants, animals and fungi) came. Although it wounds our self-esteem, eukaryotes are merely one of the branches on the tree of prokaryotes, which embedded itself in the biosphere created by bacteria. Yet eukaryotes are a very aggressive branch: wherever they find suitable conditions, bacterial ecosystems are destroyed. These relicts exist only where the concentration of salts or toxins becomes unbearable for the group now dominant (like the cobalt salts in the springs of Chihuahua). The Proterozoic is characterized by the formation of layered rocks called stromatolites (literally "stone carpets", photo 1). In the last century, modern ecosystems that produce such structures were discovered. These are bacterial mats that, as they grow, deposit layers of mineral substances in their substrate. As can be seen in photo 2, outwardly they resemble slimy lumps enveloping a hard base. Their composition includes representatives of bacterial groups that are far more distant relatives than a human and, say, a eucalyptus! To describe such bacterial communities a different approach is needed than for describing eukaryotic organisms. Both a human and a eucalyptus are clones of the descendants of a single cell (a fertilized egg). As such organisms develop, their cells acquire functional specialization and corresponding structural features, but biochemically they function practically identically. Unlike an organism, a stromatolite is a multispecies system. Cells almost identical in external structure may have fundamentally different types of metabolism. Many of the stromatolite-forming bacteria cannot be grown on an artificial medium in a Petri dish — they do not want to live outside the bacterial mat. And their analogues changed the atmosphere of the early Earth and may now influence the gaseous envelopes of other inhabited planets! Even the simple identification of one or another component of a bacterial mat is a nontrivial task, requiring, for example, the use of DNA probes that selectively bind to the genetic apparatus of the sought bacterial cells. And the study of their physiology in the mat, in an extraordinarily diverse surrounding, is additionally complicated: the biochemical processes there may be far more complex than inside an integral ecosystem inhabited by plants, animals and fungi. It is this incredible complexity that NASA decided to make sense of. Well, even if they don't find aliens, at least they'll help the development of microbiology! Amber immortality A few years ago a miner from the Mexican state of Chiapas found a piece of amber with a frog immured in it. The find ended up with a collector, who after some time consented to its scientific study. Now Mexican biologists have established that the frog's age is 25 million years. Approximately, it lived in the epoch of the transition from the Oligocene to the Miocene. At that time the world was entirely different. North America had not yet met South America, Africa was only beginning to "ride up" onto Eurasia, forming the Alps, and the result of the collision of India and Asia was the appearance of the Himalayas.

The Earth was inhabited by incredible rhinoceroses, the indricotheres (the largest land mammals), and small horses that lived in forests. In Africa at that time the most ancient anthropoid apes appeared — not large arboreal dwellers with a short tail, similar to Aegyptopithecus. And frogs, at least outwardly, have hardly changed since then! The problem of the uneven rates of evolution has still not found a final solution. The notions, characteristic of classical Darwinism, of slow and practically continuous changes of significant traits have gone into the past. As one biologist said, the life of species, like the life of soldiers, consists of long intervals of boredom (relative stability) and short periods of fear (rapid changes). Moreover, the changes themselves differ in their essence. Those connected with the appearance of new body plans (constructive solutions), oddly enough, proceed relatively quickly, whereas stable and successful variants are polished (sometimes faster, sometimes slower) over a long time. The evolutionary "age" of any creature is one and the same — from the origin of life on Earth to our days. But different branches of the tree of life passed through the crucible of evolutionary change at different times. Frogs, you see, were "reforged" quite long ago… In natural ecosystems, especially forest ones, extinct organisms have almost no chance of getting into the geological record. A dead living creature returns the material from which it is made to the ecosystem's cycle. Only the "lucky ones" who got into an extraordinary set of circumstances (for example, fell into tree resin) can escape the common fate. Finds of vertebrates in amber are extraordinarily rare and probably can give much valuable information. How good it would be if the amber's owner allowed a little hole to be drilled in it and a DNA sample of the ancient frog to be taken! Genetic individuality preserved over 25 million years could tell a great deal — if it has survived. Unfortunately, even the action of the radiation background over such a span may be enough to erase the molecular "memory". Over enormous spans of time all creatures disappeared without a trace or left behind modest remains (teeth, bones, tracks in moist soil). Somewhere deep beneath a layer of sedimentary rock are hidden the priceless finds of the future, which (if all goes well) may advance our understanding. Unfortunately, this is a fragmentary and limited resource. A substantial part of the past has disappeared without a trace… We owe them "I'll never believe it," said one worthy lady on seeing a giraffe at the zoo. The palaeontological record preserves traces of even more incredible creatures. To recreate their appearance, relying on the available remains, is no easy task, since often there is nothing to compare them with. Recall how dinosaurs differ in old and modern reconstructions. They simply began to be compared not with lizards, but with elephants, rhinoceroses, giraffes and ostriches! Some of the extraordinarily difficult creatures to reconstruct are the pterosaurs, the flying reptiles. Is it fair to compare them with birds and bats? The larger the animal, the harder it is for it to take off.

For example, a tiny aphid (a "sack" with wings) flies quite successfully, whereas a bird the size of an albatross (3.5 m wingspan, a little over 20 kg in weight) must wholly subordinate its structure to the task of rising into the air. A little spider flies by releasing a thread of web into the air — try to rise by unwinding a coil of rope! As an animal's size increases, the area of its wings and the muscular force connected with the cross-section of its muscles grow in proportion to the square of the linear dimensions, while the weight, which depends on the body's volume, grows much faster, in proportion to the cube of the dimensions. Therefore the larger a flying animal, the more economically and "engineeringly" it is constructed. The record-holder among flying birds is Argentavis, a giant relative of the condors that lived in America in the Neogene period. Its wingspan was 7.5 m. In pterosaurs the span reached 12, and possibly even 15 m — these were the most perfect flying organisms. How is one to reconstruct such creatures? For example, we know that they maintained a constant temperature, since small pterosaurs were covered with fur. And how did they reproduce — by laying eggs, like birds, or by giving birth to live young, like bats? Until recently, pterosaur eggs were unknown. And then, at a short interval, eggs of flying reptiles were finally found in China and Argentina. The Argentine find is especially interesting — the remains of a colony of small pterosaurs, Pterodaustro, about 100 million years old. These fantastic creatures had a long, upward-curved beak in which thousands of fine teeth were located (see fig.). Fortunately, it is easier for us to understand the way of life of Pterodaustro thanks to its modern analogue — the flamingo. The pterosaurs lived on the shore of a salt lake and, like flamingos, fed on algae and aquatic invertebrates that they filtered from the water with their beaks. So the authors of the BBC's "Walking with Dinosaurs" were not mistaken in reconstructing pterosaur colonies by analogy with modern bird colonies. And imagine that humans had not arisen on Earth and there were no one to visualize our planet's past. Then the extinct species would have disappeared once and for all… Distinguished relatives The giant deer (Megaloceros giganteus) is considered one of the most impressive fossils. This enormous animal, which reached three and a half metres in antler span, went extinct only 8 thousand years ago and served as one of the objects of ancient human hunting. It is hard to find a more vivid example of the wastefulness of sexual selection: there is no doubt that the magnificent antlers of the males, intended for tournament fights and attracting females, greatly hindered them in everyday life. The extinction of this species is probably connected both with the activity of our ancestors and with the spread of dense forests, through which the owner of the largest antlers in the history of life simply could not break. Although this species became a classic example of the action of sexual selection, its genealogy remained until recently unclear. The situation was corrected by researchers of University College London. The study of mitochondrial DNA obtained from fossil remains showed that the closest relatives of Megaloceros are not the deer themselves (Cervus), but the far smaller and more modest fallow deer (Dama dama).

These not-large animals retain their "childish" spotted colouring throughout life. It is notable that fallow deer have a relatively powerful spine, which allows them to carry larger and heavier antlers than those they have now. Inspired by their success, the British researchers want to clarify the family ties of the dwarf human, Homo floresiensis, found in Indonesia last year. Perhaps it is not only deer who will have to revise the notions of their genealogy. D. Shabanov. Vernanimalcula — the first of us // Computerra, Moscow, 2004. — No. 24 (548). — pp. 14–16 D. Shabanov. About our smaller brethren // Computerra, Moscow, 2005. — No. 15 (587) D. Shabanov. Amber immortality // Computerra, Moscow, 2007. — No. 8 (676) D. Shabanov. We owe them // Computerra, Moscow, 2004. — No.

47 (571). — pp. 16–17 D. Shabanov. Distinguished relatives // Computerra, Moscow, 2005. — No. 35 (607)