Animal Evolution. Genomes, Fossils, And Trees
Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the Trezona Formation of South Australia. These fossils are interpreted as most probably being early sponges. Trace fossils such as tracks and burrows found in the Tonian period (from 1 gya) may indicate the presence of triploblastic worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced today by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals. Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin is disputed, as they might be water-escape or other structures.
Animal Evolution. Genomes, Fossils, and Trees
While this textbook view of a close relationship between vertebrates and echinoderms has endured, some unexpected results from recent studies using comparisons of animal DNA to reconstruct evolutionary trees have questioned it. In parallel, some of the specific traits in how embryos develop that had been emphasised as being unique to the deuterostome branch of animals have been discovered in some species of protostomes.
To find out, we used a computer to simulate the evolution of DNA in accordance with each scenario. We started with a random synthetic DNA sequence representing an ancestral animal. This DNA sequence was allowed to evolve by accumulating mutations according to each of the three trees.
Darwin had the greatest foresight. By comparing the fossils,he brought from Galapagos, he saw the evidence of evolution.Planetology is the study of layers of rocks to trace the evidenceand ecology of plants and animals from the distant past to thepresent day. Most fossils are found in the sedimentary rocks andclay deposited on the layers of rocks. One layer deposited on the topof other. Trapped in these layers are millions of years old fossil atvarious stages of evolution. As the rivers dried up, the sedimentaryrocks become hard. The sedimentary rocks unfold like pages of agigantic book. The earliest fossil of simple structures is found in thelowest or the oldest layers. As he examined younger and youngerrocks, he found complexity of structures. No human bones wereever found in any of these ancient rocks. During the pre-Cambrianera, about 450 million years ago when the climate changed, theCambrian explosions occurred when the frozen Earth began towarm.The single cell living creature instead of growing by asexualreproduction began to grow by sexual reproduction. The interactionof two separate chromosomes resulted in variations in gene poolwhich led to divergence of life forms and evolution from the simplestto the more complex life forms called the Cambrian Explosion of life.The progeny of the recombinant genes produced complexity. Onlythose recombinant daughter cells which carry genes that producedfunctional proteins survived and the rest died. The proof of theCambrian Explosion is trapped in the fossil record which lastedfor about 25 million years. Extracting fossils from the ancient,eroded rocks is a real challenge. The erosion of sedimentary rocksover the years is due to rain falls, windstorms, running waters, andtransportations of the rocks. Once DNA extraction is purified fromthe fossils, its genome could be sequenced, and its date could beestimated by Radioactive Dating method (Figure 2).
UCLA-sponsored astrobiologists have progressed on several fronts pertaining to ancient animal life this year. Microbial studies on early Cambrian fossils confirm that shifts in body symmetry occurred several times in animal evolution. Sense organs are a fundamental feature of animal life. Studies of the evolution of the developmental genetics of sensory and neural systems in basal animals continued this year. Our group reported work on the eye/sense organ developmental gene sine oculis from sponges, jellyfish, ctenophores and basal bilaterians, including flatworms and mollusks. The work demonstrates the presence and, in the case of jellyfish, expression of these sense organ related genes, providing critical insight into the evolution of sense organs. We continue to make progress in this area with recovery of other sense organ genes such as optix and eyes absent in basal animals. We have also been successful in the first stages of recovery of genes that we infer to be important in the evolution of the skeletons of Bilateria. This investigation of the developmental underpinnings of invertebrate skeletogenesis will ultimately allow us to integrate the fossil record of evolution. 041b061a72