Does evolution proceed toward increasing complexity?

In the approximately 3.8 billion years since life originated on Earth, evolution has resulted in many complex organisms and structures. The human brain and stereoscopic eyes are just two examples. At the same time, simpler organisms like algae, bacteria, yeast, and fungi, which arose several billion years ago, not only persist but thrive. The presence of single-celled organisms alongside complex organisms like humans testifies to the fact that evolution within a given lineage does not necessarily advance toward increasing complexity. When more complex organs are advantageous, complex organs have arisen. Single-celled organisms, however, fill many roles, or niches, much better than any multicellular organism could, and so they remain in a relatively stable state of adaptation.