5. The Evolution of Metazoa

5.6 The Mammals

The asteroid impact that wiped out the dinosaurs 65 million years ago allowed the mammals to inherit the earth. The emergence of the mammals was not due to the fact that they had a superior intelligence and were able to eliminate the dinosaurians. They had to wait 100 million years for their chance. Their rebirth was due to specialized insectivores, the pantotherians. Europe was at that time a very different place: a tropical island archipelago connected by high-latitude land bridges to North America separated by seaways from Africa and Asia. The continent was a land of revolutions.

5.6.1 Insects

Angiosperm plants i.e. all flowering plants, grasses, vegetables and most trees, with the exception of conifers, appeared as early as the end of the Devonian period but became fantastically successful only at the end of the Jurassic period. A riot of flowering plants burst upon the world some 65 million years ago. Darwin called their sudden appearance "an abominable mystery" that has perplexed evolutionary biologists ever since. Since the closest known relatives of angiosperms, i.e. the conifers, are woody, it was assumed that flowering plants arose from woody plants resembling the magnolia tree. This idea was strengthened by the discovery that the most primitive living angiosperm is Amborella, a woody shrub in New Caledonia. The 125 million-year-old fossil Archaefrutus sinensis found in 2002 13 indicates that the first angiosperms were in fact fast-living herbs that lived in the water. There, free from competition from other seed plants, early flowering plants that still lacked sepals and petals could have bloomed into new shapes.

Besides the possible role they played in the elimination of the dinosaurs, their success was essential on at least two accounts. Firstly, they favored the emergence of arboreal life: the emergence of a successful arboreal life is hardly conceivable among spruce- or fir trees but can be initiated among leaves-bearing ramified trees. Secondly, angiospermic plants produce flowers. An abundance of flowers and fruits spelled the success of the insects, which exploited them the best. There are 33 insect orders, the last one being discovered in 2002 (Mantophasmatodea, fig. 5.25); it consists of 3 species each represented by a single or two or three individuals!

Figure 5.25. Mantophasmatodea. The upper picture represents the “gladiator”, living in Namibia. O. Zombro discovered this modern representative of the Order Mantophasmatodea in March 2002. Only two or three such insects have been found until now. The lower picture represents a fossil species of the same Order, conserved in amber, which lived during the Cretaceous.

One other order has less than 20 members, while another has more than 300,000 species. So far, 750,000 insect species have been placed into 33 orders.

5.6.1.1 Primitive insects

The earliest insects cleaved from their nearest relatives, the fairy shrimps, about 430 million years ago, close to the emergence of the first land plants. The insects were the undisputed masters of the air during the 300 million years that lasted from the Devonian period to the Cretaceous period. The first insects known through fossils were deprived of wings. They very probably arose from a myriapod ancestor. Insects were the first group of animals to fly, more than 100 million years before reptiles and birds, and this trait is widely seen as a key evolutionary innovation underlying their spectacular success. The first flying insects appeared during the Primary era. These creatures were big, such as giant dragonflies (Odonatas), which had only to slowly move their wings in order to fly. Their main concern was probably to glide from one tree to another, avoiding ground level where roamed the batracians. The wing muscles were very probably modified leg muscles and they were under the direct control of the central nervous system. In other words, these insects were endowed with a synchronous muscular system.

When the flying muscles are in synchrony, an activity superior to about 100 movements per second is impossible under penalty of tetanization. At higher speeds, the system jams. With such a system, the yield is similar to the one obtained by the muscular contractions of vertebrates, including man and humming-birds, which attain a frequency of wing movements of only 90 per second. Flying insects obeying to such a synchronized control are Orthoptera (grasshoppers), Odonata (Dragon fly, Damselfly) and Lepidoptera (butterflies). These insect groups are primitive and have evolved little over the last 300 million years. They are in general big, their wings are large compared to their bodies and they are usually excellent gliders.

5.6.1.2 Evolved insects

A superior activity is only possible when the nerve-muscle system is uncoupled and made asynchronous, that is, one nervous impulsion commands several muscular contractions. The frequency of movements may then increase to 1000 per second. Everything points to the fact that this asynchronous muscular system appeared independently in many insect groups that had already separated for a long time. This original solution to the problem of flight by organisms heavier than air appeared during the Secondary era. During this era, insects diversified with the appearance of flower plants. All flowers are not necessarily big and for a symbiosis such as the one achieved by a great number of insects and plants, a reduction in the size of the insect was a must. However, a small insect flies well only if it can rapidly move its wings, and its adaptation to small size must by necessity have been pursued by an adaptation to an asynchronous muscular contraction. Apparently, this was the only solution available and all the insect groups that evolved towards a snug adaptation to the exploitation of the advent of flower plants adopted it. Among these groups, one counts the Diptera (flies, mosquitoes), the Hymenoptera (ants, bees, wasps), the Hemiptera (true bugs and water scorpions) and the Coleoptera (beetles, fireflies), which represent the largest Order of animals in the world. It is of course not excluded that later, several evolutive lines, having acquired the asynchronous character, reverted towards a bigger or gigantic size, especially among the tropical species.

Ant queens initially have wings but shed them after mating when they begin to establish a new colony. Workers in contrast are born wingless. Worker ant winglessness evolved probably only once. This evolutionary reversal is thought to have given ants the mobility to search for food sources in the ground, and thus figures prominently in their overwhelming ecological dominance. Ants constitute 10% to 15% of the entire animal biomass in most terrestrial environments.

5.6.2 Insectivorous mammals

The success of the insects favored the appearance of insectivorous mammals. Traces of these insectivorous pantotherians are found during the 10 million years preceding the end of the Jurassic period. The pantotherians belonging to the architerian group were thus present 40 million years before the detectable presence of the marsupials and insectivores, which are at the origin of the present-day mammalian fauna.

The pantotherian insectivores were able to survive during the dinosaurian dominance because they were small and occupied an ecological niche that the dinosaurs were unable to fill. These reptiles were active during daytime only, whereas the pantotherians enjoyed night vision. They fed on insects and fruits, which appeared on the scene during the Jurassic and Cretaceous periods. These fuels are extremely rich and may have helped the pantotherians on their way towards homeothermy, since generosity in the expenditures of energy was thereby allowed. It is curious that profusion is inscribed within the individual cells of homeotherms: whereas the cells of insects, fishes, amphibians and reptiles produce a ribosomal RNA that will be almost entirely integrated in the ribosomes (see fig 3.1), the cells of birds and mammals produce a ribosomal RNA almost twice as big as the one needed, so that half of this RNA is lost when incorporated into the ribosomes. We have seen earlier that RNA processing is a potent adaptive help to changing environments. On this account and several others, birds and mammals are very alike and different from reptiles. It could be that birds are not descendants of the dinosaurs (the Coelurosaurs) but well a diverging branch of very primitive mammals. Yet, their brain is so much different from that of mammals and so alike that of reptiles, that this hypothesis is difficult to retain (see fig 6.11).

5.6.3 The evolutive lines

Originating from the synapsids, five mammalian groups appeared, of which three recessed. Remnants of one of these groups, the monotremes, are found in Australia with the echidna and the ornithorinque that is provided with a beak, lays eggs, breast-feeds its young and the male of the species wears poisonous ergots. The homeothermy of these animals is only weakly established. The two mammalian groups that prospered were the marsupials (kangaroos, opossums) and the eutherians (rats, whales, lions, apes, rabbits, horses, dogs etc.), which are truly placentary. These two groups started on divergent lines at the end of the Jurassic period, about 175 million years ago, and this brings the split back to the time when the Pantotherians were on the scene. The chances of survival of these two mammalian groups were initially identical. The marsupials eclosed in North America. The Pangea break-up was at that time not complete and the marsupials expanded in South America, Antarctica and Australia. The cold eliminated the marsupials in Antarctica and eutherian carnivores eliminated them in the Americas. The few marsupials that invaded Eurasia were also eliminated by the eutherians.

The most ancient placental mammal yet discovered is Eomaia. It is a shrew-sized creature weighing about 20 grams that lived 125 million years ago. The placental mammals subdivide into four distinct groups. The oldest group, the Afrotheria, originated in Africa about 130 million years ago. The group includes elephants, aardvarks, elephant shrews and golden moles. The split of Africa and South America was the occasion for the Xenartra to appear in South America, about 100 million years ago. The group includes anteaters, armadillo’s and sloths. The third group, with the forbidding name “Euarchontoglires” is much larger. It includes rodents, rabbits, tree shrews and primates, and originated in the Northern hemisphere. A bridge must have existed for a long time between North America and Europe, since at least one primitive primate genus, Plesiadapis, has been found in Pleistocene deposits of France and Colorado. The last group, a sister group of the Euarchontoglires, was the Laurasiatheria, also originating in the Northern hemisphere about 75 million years ago. It is composed of whales and dolphins, hippos and cows, pigs and llamas, rhinos and tapirs, and also horses, bats, cats and pangolins. The superiority of the eutherians became manifest only late during the Tertiary era, when they started to bloom into various different orders (fig. 5.26).

Figure 5.26. Modern mammalians. The diagram shows the breadth of family representation among four main groups of mammals over the last 60 million years.

The sudden appearance, about 50 million years ago, during the early Eocene, of artiodactyls (pigs, camels, deer, cows, caribou’s etc.), of perissodactyls (horses, tapirs, rhinos) and of primates (lemurs, tarsiers, monkeys and apes) in abundance, with little indication of their ancestry, presents one of the great enigmas of mammalian history. Where and how did they originate? This radical reshuffling of earth’s biota coincided with a brief but intense episode of global warming about 55 million years ago, at the Paleocene/Eocene boundary. The biological effects of the fleeting climatic perturbation was a wave of anatomically modern groups of mammals, primates, artiodactyls, perissodactyls and opossum-like marsupials that showed up en masse at the expense of archaic forms that became extinct. The newcomers, in North America and Europe, differ so fundamentally from Paleocene mammals that they could not have evolved in situ. They show up about 10,000 years after the peak of the global warming at the Eocene boundary. In Asia (China’s Hunan Province), the same animal species are present before the global warming, in the Paleocene. Asia is the birthplace of numerous modern groups of mammals, including our own order, the Primates.

Despite the much-heralded ability of evolutionary trends to fill in all available ecological niches, there was among the marsupials no successful trend similar to that of the eutherian aquatic carnivores (seals) and cetaceans, and arboreal primates. The female marsupials carry in general their embryo within a ventral pocket, at a very early stage of embryonic development. Development and suckling continue there. Yet, arboreal life presupposes the clinging of the young to the mother’s fur. For this to be possible, the young must have an already fairly well developed central nervous system, which is not the case with the Marsupials. This precludes any serious marsupial attempt in that direction, although the opossum did well. The opossum that successfully invaded North America from the South through the Isthmus of Panama is a very primitive species, since it possesses 50 teeth and this is not much less than have the reptiles.

If the Pangea break-up had occurred later, one may imagine that the marsupials would have largely eliminated primitive eutherians. An extreme consequence of this would have been that the most intelligent mammalian species (killer whales, dolphins, porpoises, chimpanzees, gorillas) would be absent from the world scene. Man a fortiori also.

References

13. Ge Sun et al. Archaefructaceae, a new basal angiosperm family. Science 296: 899-904, 2002

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