Simple life forms are more likely to exist just because they are simple. Bacteria were the only life forms found in the oldest known fossils (3.5 billion years ago). Bacterial fossils are hard to find, because they are so small and do not have hard structures. But, spores and blue-green algae are common fossils because they have a hard cell wall that is more easily preserved as fossils.
Bacteria have always dominated life on
earth and do so even today in the sense that they have more
individuals and live in practically all environments, including hostile
environments in frozen areas, in hot springs, and deep within the earth
and deep in the ocean.
Complexity Is Inevitable
We expect life forms more complex than bacteria because life can only evolve in that direction - it can't get any simpler. The late professor Stephen Gould (see Story Time in Populations) believed that evolution is not driven toward progress nor toward the creation of higher forms such as humans. Maybe higher forms just happen because that is the only direction in which evolution can occur. But that is not true. Many parasites appear to have evolved from higher forms by having traits that enabled them to thrive as parasites.
What Dr. Gould missed was that there is an advantage to having complex
nervous systems, which has put humans at the top of the mammal group and
insects at the top of the invertebrates. These advanced forms were not
accidents. Rather, they are at least partly predictable because
complex nervous systems provide great advantages to occupy multiple niches and to defend against predators.
The Virtues of Sex
We even define a species in terms of a group of like animals (or plants) that are sexually isolated. What does that mean to you? To scientists it means that members of one species cannot breed with members of another species and produce offspring that can reproduce with that same group.