...complexity of life on Earth? It is an observable fact that, ever since life began roughly 3.9 billion years ago, there has been a sustained and accelerating increase in the maximum complexity of living organisms. That is to say, although the average biological complexity of all life may not have changed dramatically, the complexity of the “leading organisms” (the most complex life forms existing at any given time) has generally increased at an accelerating pace.

For example, humans, which are currently the most biologically (although not genetically) complex species on Earth, are far more complex than any other species that has ever existed in the history of our planet. However, we are still vastly outnumbered by simple organisms such as insects and bacteria, so the average complexity of all living species is not much greater than it was in the past.

The timeline of evolution clearly shows this accelerating increase in the upper bound of biological complexity:

3.9 billion years ago, there were only simple cells (prokaryotes);
2.1 billion years ago, the first complex cells (eukaryotes) appeared;
1 billion years ago, multi-cellular life emerged;
600 million years ago, simple animals (sponges) appeared;
500 million years ago, the first vertebrates (fish) developed;
360 million years ago, amphibians emerged;
300 million years ago, reptiles appeared;
220 million years ago, mammals developed;
58 million years ago, the first primates evolved;
15 million years ago, the great apes appeared;
4 million years ago – Australopithecus afarensis, first bipedal hominid;
1.8 million years ago – Homo erectus, ancestor of modern humans;
195,000 years ago – Homo sapiens appear.

This is an interesting trend, because evolution itself has no innate bias towards complexity. The only thing natural selection does is that it favors those members of a population that have the genes most suited to helping them survive in their particular environment long enough to reproduce.

The methods through which biological complexity increases are certainly well understood (for example, gene duplication and subsequent mutation). There is nothing about such an increase that contradicts evolution. The question, however, is not how this happens, but rather why, and especially why to such an astonishing degree. Against all odds, throughout the history of life, natural selection has seemed to favor steady and accelerating increases in maximum complexity whenever possible, again and again, as if this process is not just a side effect of evolution, but rather an independent natural drive or tendency. In order to make this increase in complexity sustainable and overcome its initial drawbacks, natural selection has further selected for a great diversity of intricate and complex adaptations.

What known processes could possibly account for this?

Any comments, ideas, references, etc. are welcome.
Under the right conditions, natural selection certainly can lead to more complex organisms, but only if such an adaptation is beneficial to their survival. However, an increase in biological complexity would usually be detrimental, for several reasons.

One is that an overall increase in complexity also requires more complex body systems to sustain the organism, which makes living a lot less efficient. Bacteria, for example, are very simple and resilient and can survive in virtually any environment, whereas more complex life forms can only survive in very specific environmental conditions.
In addition, an increase in complexity is partly correlated with an increase in size, which of course requires greater food and energy intake, as well as increasingly more complex and specific foods and more diverse nutrients to sustain the organism’s already complex body systems. Simple organisms have very modest requirements for survival, needing little food and energy, and usually only a few simple nutrients that are readily available.

Another reason why greater complexity is harmful for the survival of an organism is that, at least to an extent, greater biological complexity implies a larger, more complex genome, which makes rapid adaptation more difficult in the case of radical environmental change. This is why single-celled organisms can evolve so quickly and become resistant to antibiotics, whereas larger, more complex organisms require significantly longer periods of time in order to accumulate enough genetic changes to produce noticeable biological adaptations.
For more complex organisms, there is also greater risk of damage or error, simply because there are more things that can go wrong both at the molecular and anatomical levels.

What are the advantages of increased complexity? One idea would be that, since increased complexity also usually causes at least some increase in size, this allows organisms to cover vaster and more diverse areas more quickly and increase chances of survival. This argument, however, is flawed, because smaller organisms can reproduce more rapidly, making up for their small size. Plus, an increase in size brings all of the aforementioned problems.
Another hypothesis would be that increased complexity allows for new, more diverse adaptations, such as intelligence, which greatly aid survival. Yet for genetically complex creatures, these adaptations take far too long to evolve to be of any initial benefit. And these complex adaptations would not be necessary if the organism was simpler and had fewer survival requirements in the first place. Besides, complexity would have to arise first in order for these adaptations to be selected. The mere fact that they have to potential to be beneficial cannot cause life to increase in complexity beforehand. Evidently, any advantages that increased complexity might bring would be greatly outweighed by its drawbacks.