How do single celled organisms survive

How these extremely simply built creatures manage to do this was not entirely clear until now. However, a research team at TU Wien Vienna has now been able to simulate this process on the computer: They calculated the physical interaction between a very simple model organism and its environment.

This environment is a liquid with a non-uniform chemical composition, it contains food sources that are unevenly distributed. The simulated organism was equipped with the ability to process information about food in its environment in a very simple way. With the help of a machine learning algorithm, the information processing of the virtual being was then modified and optimised in many evolutionary steps. The result was a computer organism that moves in its search for food in a very similar way to its biological counterparts.

This is called chemotaxis. The behaviour of other, multicellular organisms can be explained by the interconnection of nerve cells. But a single-celled organism has no nerve cells -- in this case, only extremely simple processing steps are possible within the cell. Until now, it was not clear how such a low degree of complexity could be sufficient to connect simple sensory impressions -- for example from chemical sensors -- with targeted motor activity.

Our single-celled organism consists of three masses connected by simplified muscles. The question now arises: can these muscles be coordinated in such a way that the entire organism moves in the desired direction? And above all: can this process be realised in a simple way, or does it require complicated control?

King hypothesizes that something similar could have happened more than million years ago, when the last common ancestor of all animals started its fateful journey toward multicellularity. Later, multicellularity became fixed in the genes as a developmental program. The first bacteria may date back as far as 3. But animals, the first complex multicellular life form, took much longer to emerge.

Corals, sea squirts, sponges and tube worms all begin life as larvae floating in the water, and other research teams have shown that they too respond to compounds released by bacteria as signals to attach themselves to rocks or other surfaces and transition to a new life form.

If this kind of relationship is so common among animals from the most ancient families, it seems plausible that the first animals were equally attuned to their bacterial neighbors. Figuring out how, exactly, the bacteria trigger this response will help clarify whether they played a similar role long ago. What triggered the explosion of complex multicellular life in the Cambrian period? Increased oxygen undoubtedly had something to do with it — prior to a period sometime before million years ago, atmospheric oxygen levels were too low to diffuse easily into organisms with multiple layers of cells, limiting the size of all life forms.

But an increase in oxygen is probably not the whole story, said Andrew Knoll , a professor of earth and planetary sciences at Harvard University. Once oxygen levels rose past this low level, predation likely provided a strong incentive for animals to get bigger and more complicated, and to develop new body plans. It was an ecological arms race of size and complexity: Bigger predators have an advantage in catching prey, while larger prey can more easily avoid being eaten. The need to escape or repel predators also likely inspired the first scales, spines and body armor, as well as some of the wilder body plans seen in Cambrian fossils.

Historically, photosynthetic bacteria pumped oxygen into the oceans for billions of years, setting the stage for complex multicellular life. And according to the endosymbiotic theory , proposed in the 20th century and now widely accepted, the mitochondria inside every eukaryotic cell were once free-living bacteria.

At some point more than a billion years ago, they took up residence inside other cells in a symbiotic relationship that endures in nearly every animal cell to this day. In their role as dinner, bacteria also likely provided raw genetic material for the first animals, which probably incorporated chunks of microbial DNA directly into their own genomes as they digested their meals.

In her view, animals should rightly be considered host-microbe ecosystems. Several years ago McFall-Ngai, along with Hadfield, convened a broad group of developmental biologists, ecologists, environmental biologists and physiologists, including King, and asked them to formulate a microbial manifesto — a declaration of bacterial significance.

The paper , which appeared late last year in the Proceedings of the National Academy of Sciences, cites evidence from many corners of biology to argue that the influence of microbes on the origin, evolution and function of animals is pervasive and essential to understanding how animal life evolved. The biology of choanoflagellates resembles that of animals in other unexpected ways, King found. The sequence revealed genes for dozens of sections of proteins that also appear in multicellular animals, where they help cells stick together and also guide development and differentiation.

What are they doing in single cells? In single-celled organisms, signaling allows populations of cells to coordinate with one another and work as a team to accomplish tasks no single cell could carry out on its own. Essentials of Cell Biology. What Are the Essent Prev Page. Next Page.