The origin of animals is also one of the more mysterious episodes in the history of life. Changing from a single-celled organism to a trillion-cell collective demands a huge genetic overhaul. The intermediate species that might show how that transition took place have become extinct.The trick of building a multicellular animal out of a single cell predecessor isn't in getting cells to stick together. It is in handling problems that single cells never face:
“We’re just missing the intervening steps,” said Nicole King, an evolutionary biologist at the University of California, Berkeley.
To understand how animals took on this peculiar way of life, scientists are gathering many lines of evidence. Some use rock hammers to push back the fossil record of animals by tens of millions of years. Others are finding chemical signatures of animals in ancient rocks. Still others are peering into the genomes of animals and their relatives like Capsaspora, to reconstruct the evolutionary tree of animals and their closest relatives. Surprisingly, they’ve found that a lot of the genetic equipment for building an animal was in place long before the animal kingdom even existed.
“When cells die in a group, they can poison each other,” said Dr. Michod. In animals, cells die in an orderly fashion, so that they release relatively few poisons. Instead, the dying cells can be recycled by their living brethren.Fascinating stuff.
Another danger posed by multicellularity is the ability for a single cell to grow at the expense of others. Today that danger still looms large: cancer is the result of some cells refusing to play by the same rules as the other cells in our body.
Even simple multicellular organisms have evolved defenses to these cheaters. A group of green algae called volvox have evolved a limit to the number of times any cell can divide. “That helps reduce the potential for cells to become renegades,” said Dr. Michod.
To figure out the solutions that animals evolved, researchers are now sequencing the genomes of their single-celled relatives. They’re discovering a wealth of genes that were once thought to exist only in animals. Iñaki Ruiz-Trillo of the University of Barcelona and his colleagues searched Capsaspora’s genome for an important group of genes that encode proteins called transcription factors. Transcription factors switch other genes on and off, and some of them are vital for turning a fertilized egg into a complex animal body.
And for all those religious nuts who keep denying evolution because they want "intermediates", the fact is evolution is full of spandrels which lay the foundations for an evolutionary leap:
Studies by other scientists point to the same conclusion: a lot of the genes once thought to be unique to the animal kingdom were present in the single-celled ancestors of animals. “The origin of animals depended on genes that were already in place,” Dr. King said.Meanwhile the origins of life get pushed back billions of years. And here's the latest thinking on the origin of animals:
In the transition to full-blown animals, Dr. King argues, these genes were co-opted for controlling a multicellular body. Old genes began to take on new functions, like producing the glue for sticking cells together and guarding against runaway cells that could become tumors.
Last year, Adam Maloof of Princeton and his colleagues published details of what they suggest are the oldest animal fossils yet found. The remains, found in Australia, date back 650 million years. They contain networks of pores inside of them, similar to the channels inside living sponges.There's a lot more fascinating details in Carl Zimmer's article. Go read the whole thing.