![]() Bacteria get caught up in the current and stick to the collar, and the choano engulfs them. The tail, thrashing back and forth, drives a current across a rigid, collarlike fringe of thin strands of cell membrane. Choanoflagellates are not the most charismatic of creatures, consisting of an oval blob equipped with a single taillike flagellum that propels the organism through the water and also allows it to eat. To understand why this might have happened the way it did, King began studying choanoflagellates, the closest living relative to animals, nearly 15 years ago as a postdoc at the University of Wisconsin, Madison. But animals were the first to develop complex bodies, emerging as the most dramatic example of early multicellular success. ![]() Nonetheless, during the course of evolution, the transition to multicellularity happened separately as many as 20 different times in lineages from algae to plants to fungi. And to her surprise, she found that bacteria may have played a crucial role in ushering in this new era. In these organisms, which can live either as single cells or as multicellular colonies, she has found much of the molecular toolkit necessary to launch animal life. “Choanoflagellates are to my mind clearly the organism to look at if you’re looking at animal origins,” King said. So King turned to choanoflagellates, microscopic aquatic creatures whose body type and genes place them right next to the base of the animal family tree. Fossils don’t offer a clear answer: Molecular data indicate that the “Urmetazoan,” the ancestor of all animals, first emerged somewhere between 600 and 800 million years ago, but the first unambiguous fossils of animal bodies don’t show up until 580 million years ago. How did life make this spectacular leap from unicellular simplicity to multicellular complexity? Nicole King has been fascinated by this question since she began her career in biology. Every animal that has lived since then has been a variation on one of the themes that emerged during this time. Prototypes for every animal body plan rapidly emerged, from sea snails to starfish, from insects to crustaceans. Soon afterward, roughly 540 million years ago, animal life erupted, diversifying into a kaleidoscope of forms in what’s known as the Cambrian explosion. These complex multicellular creatures were the first animals, and they were a major success. ![]() They developed new ways for cells to communicate and share resources. They began to divide up the labor of life, so that some tissues were in charge of moving around, while others managed eating and digesting. Cells organized themselves into new three-dimensional structures. * Original story reprinted with permission from Quanta Magazine, an editorially independent division of whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.*Then, more than 3 billion years after the appearance of microbes, life got more complicated. Some microorganisms attempted multicellular arrangements, forming small sheets or filaments of cells. For billions of years, single-celled creatures had the planet to themselves, floating through the oceans in solitary bliss.
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