Have you ever imagined which living creature was the first to be on Earth? Well, the answer is it was a unicellular organism, probably a bacterium. Unicellular organisms also known as single celled organisms, are believed to be the oldest form of life on Earth. These are organisms that consist of a single cell, unlike a multicellular organism that consists of multiple cells. Unicellular organisms do not have conscious thought, the sort you find in humans and other complex animals, due to the fact that single celled organisms do not have a nervous system.

It was believed that due to the fact that single celled organisms have neither a brain nor a nervous system, they do not have the ability to make complex decisions or the ability to memorize. However, scientists say they have recently discovered what they are calling signs and symptoms of complex decision making in a single celled organism, giving a new existence to an idea that was brushed off and not taken seriously over a century ago.

The single celled organism, named Stentor roeselii, is what sparked the interest of the scientists. This aquatic unicellular organism is a free-living ciliate species of the genus Stentor. It is a common and widespread protozoan, found throughout the world in freshwater ponds, lakes, rivers and ditches.

Scientists have found that Stentor roeselii responds differently over a period of a time to the same stimulus. This particular ability of the organism is evidence that it can make choices, or at least does whatever the single-celled equivalent of changing one’s thoughts are. It is not completely accurate to say that a creature without any type of nervous system is actively thinking, however, the discovery challenges many of scientists’ assumptions of animal intelligence.

Scientists started their research on the single celled organism to understand its behavior better. Multiple unicellular organisms were put through a ‘Poke Test’ where they were provided with the same stimulus over a period of time. It was observed that other organisms regularly responded less to repeated stimuli. Stentor roeselii, however, first bent far from the source of the stimulus, but later changed its procedures and flapped its cilia in defense, contracted, or floated away. This behavior is different from other similar single celled organisms.

The probability that the organism would pick one method over some other was almost a 50-50 split, suggesting that some biological mechanism is selecting one over the other almost like it is flipping a coin to choose. Saying that Stentor roeselii can make choices is a vague, rather than precise, explanation. However, until scientists keep probing the complex behavior of the organism, this may be the most accurate way to explain what is going on.