Evolutionary enigmas | Life | Science News
When scientists draw evolutionary trees, they compare and contrast traits for clues on how animals are related. In general, biologists favor the simplest solution usually the one in which most lineages radiating out from a common ancestor share most of the ancestors features. This concept of simplicity, called parsimony, has long guided thinking on animal origins.
All animals alive today descended from a clump of cells that were able to communicate and adhere to one another more than 800 million years ago. This event appears to have happened once, as did other milestones in animal evolution such as the organization of cells into tissue layers, says Claus Nielsen, a biologist at the Natural History Museum of Denmark in Copenhagen and the author of the textbook Animal Evolution.
In traditional trees of life, the sponges branch off first, as multicellular animals without much specialization. Jellyfish, sea anemones and corals are thought to come later, from an ancestor with multiple cell types, and some cells organized into an outer layer of tissue surrounding the body and an inner tissue layer lining the gut. An animal with all these features plus nerve cells, a rudimentary brain and a middle tissue layer that forms muscles is traditionally thought to have given rise to comb jellies and the rest of the animals.
With the earliest animal lineages arranged in this order, major transitions paved the way for further innovations. This is evident not only in body structures that look alike, but in shared molecular underpinnings. In the case of multicellularity, many of the same proteins stick cells to one another and communicate messages between cells in all living animals. The same concept holds true for muscles and the central nervous system, which consist of several distinct parts built by networks of proteins encoded by genes. The fact that many of the interacting components are shared by all animals leaves Nielsen and many others resistant to the idea that comb jellies originated the parts on their own and then converged on a common design. The more complicated a shared structure, the less likely it is to be convergent, or to have evolved independently, says Nielsen. One cannot exclude the possibility of convergence, but there is a big difference between possible and probable.
In the 1990s, biologists predicted that studies of animal genomes would mirror the gradual addition of anatomical complexity in early animal evolution. Where humans have about 22,000 genes in their genome, it was expected that sponges, sea anemones and comb jellies would have far fewer. Yet in 2007, biologists were taken aback by a report in Science showing that the starlet sea anemone has nearly as many genes as a human. The genetic potential for complexity, it seemed, existed early on.
View larger image Animals develop their main tissue layers as embryos; the layers give rise to muscles, skin and organs. Biologists traditionally have believed that animals with one or two tissue layers originated before animals with three.
Comb jellies made a splash a year later. An evolutionary tree built according to similarities in select stretches of DNA, rather than shared anatomical traits, placed the comb jellies below the brainless sponges. At the time, scientists largely dismissed the finding, calling it a result of imperfect tree-building algorithms. In fact, the team initially left the finding out of its paper. But the reviewers wanted us to say something, so we noted the result and said it needed further analysis, says Andreas Hejnol,
a coauthor on the 2008 report in Nature and an evolutionary developmental biologist at the Sars International Centre for Marine Molecular Biology in Bergen, Norway. But privately among ourselves, we talked about what it would mean if [comb jellies] are at the base, Hejnol says. It would mean that they evolved complexity independently, or that the sponges lost a massive amount of complexity.
When scientists draw evolutionary trees, they compare and contrast traits for clues on how animals are related. In general, biologists favor the simplest solution usually the one in which most lineages radiating out from a common ancestor share most of the ancestors features. This concept of simplicity, called parsimony, has long guided thinking on animal origins.
All animals alive today descended from a clump of cells that were able to communicate and adhere to one another more than 800 million years ago. This event appears to have happened once, as did other milestones in animal evolution such as the organization of cells into tissue layers, says Claus Nielsen, a biologist at the Natural History Museum of Denmark in Copenhagen and the author of the textbook Animal Evolution.
In traditional trees of life, the sponges branch off first, as multicellular animals without much specialization. Jellyfish, sea anemones and corals are thought to come later, from an ancestor with multiple cell types, and some cells organized into an outer layer of tissue surrounding the body and an inner tissue layer lining the gut. An animal with all these features plus nerve cells, a rudimentary brain and a middle tissue layer that forms muscles is traditionally thought to have given rise to comb jellies and the rest of the animals.
With the earliest animal lineages arranged in this order, major transitions paved the way for further innovations. This is evident not only in body structures that look alike, but in shared molecular underpinnings. In the case of multicellularity, many of the same proteins stick cells to one another and communicate messages between cells in all living animals. The same concept holds true for muscles and the central nervous system, which consist of several distinct parts built by networks of proteins encoded by genes. The fact that many of the interacting components are shared by all animals leaves Nielsen and many others resistant to the idea that comb jellies originated the parts on their own and then converged on a common design. The more complicated a shared structure, the less likely it is to be convergent, or to have evolved independently, says Nielsen. One cannot exclude the possibility of convergence, but there is a big difference between possible and probable.
In the 1990s, biologists predicted that studies of animal genomes would mirror the gradual addition of anatomical complexity in early animal evolution. Where humans have about 22,000 genes in their genome, it was expected that sponges, sea anemones and comb jellies would have far fewer. Yet in 2007, biologists were taken aback by a report in Science showing that the starlet sea anemone has nearly as many genes as a human. The genetic potential for complexity, it seemed, existed early on.
View larger image Animals develop their main tissue layers as embryos; the layers give rise to muscles, skin and organs. Biologists traditionally have believed that animals with one or two tissue layers originated before animals with three.
Comb jellies made a splash a year later. An evolutionary tree built according to similarities in select stretches of DNA, rather than shared anatomical traits, placed the comb jellies below the brainless sponges. At the time, scientists largely dismissed the finding, calling it a result of imperfect tree-building algorithms. In fact, the team initially left the finding out of its paper. But the reviewers wanted us to say something, so we noted the result and said it needed further analysis, says Andreas Hejnol,
a coauthor on the 2008 report in Nature and an evolutionary developmental biologist at the Sars International Centre for Marine Molecular Biology in Bergen, Norway. But privately among ourselves, we talked about what it would mean if [comb jellies] are at the base, Hejnol says. It would mean that they evolved complexity independently, or that the sponges lost a massive amount of complexity.
They refuse to accept the obvious situation that simply somebody designed it 
Smart dumb nikkaz, they stay trying to fit a square into a circle.
Intresting how Blacking and his fellow posters who have invisible friends missed these parts
