The debate continues: Is Euglena a plant or an animal? This question has had a tremendous impact on the study of biology.
Most people have a good notion of the difference between plants and animals. The thought is generally that animals move about, plants do not, at least not rapidly. Plants also usually produce their own food with the use of chlorophyll, which uses sunlight to produce sugars.
Before the invention of the microscope, and even sometime afterward, this view held sway in biology. It was a major foundation of biological sciences, as a rule. The discovery of euglena must have been a shock to a lot of people, particularly scientists.
Euglena is a single celled organism. What makes it special is that like an animal, it can move around quite well, with the use of a flagellum. This is a hair-like projection that can be whipped around to produce motion. There are several single-celled animals that use a similar means. Yet, euglena also contains chlorophyll. In the sunlight, it can produce its own food, like a plant.
It gets stranger, though. In sunlight it does make its own food, but in darkness euglena has the ability to capture and eat food like a single-celled animal.
Another twist is that it doesn't reproduce sexually, as many animals do; it splits down the center, which also separates the nucleus and the DNA. Each part becomes a new euglena. One has the flagellum, the other doesn't. However, the new euglena that doesn't have a flagellum soon grows one. It is also able to detect light and will usually move toward it. This is presumably nature's way of making sure it gets sustenance.
Either way, euglena has traits of both plants and animals. It could very well be an intermediary step between the two. It is still being studied; however, it is difficult to say if this will be determined absolutely. Still, it has caused biologists to rethink their definitions of plant and animal. One day, it might prove to be that there isn't as much of a difference as people once thought.
It is easy to see why there is a debate. This creature can't be easily defined as either a plant or animal. More to the point, it could be classified as either.
The impact to biology has been that what was thought of as a clear way to tell the plant and animal kingdoms apart, might not be so clear after all. Research continues, but this isn't at all a negative. Astounding things may be learned through the research.