The chapter begins with discussion of intelligence in simple unicellular organisms followed by that of animals with complex nervous systems. Surprisingly, even organisms that do not have a central brain can navigate their complex environments, forage, and learn. In organisms with central nervous system, neurons and synapses in the brain provide elementary basis of intelligence and memory. Neurons generate action potentials that represent information. Synapses hold memory and control the signal transmission between neurons. A key feature of biological neural circuits is plasticity, that is, their ability to modify the circuit properties based both on stimuli and time intervals between them. This represents one form of learning. The biological brain is not static but continuously evolves based on the experience. The field of AI seeks to learn from biological neural circuitry, emulate aspects of intelligence and learning and attempts to build physical devices and algorithms that can demonstrate features of animal intelligence. Neuromorphic computing therefore requires a paradigm shift in design of semiconductors as well as algorithm foundations that are not necessarily built for perfection, rather for learning.

Here, I use the phrase ‘intelligent universe’ to refer to all intelligent entities everywhere. Whether the set of all such entities overlaps with, or is a subset of, the biological universe depends on whether we include artificial intelligence in it. I focus here on biological intelligence. On Earth, evolution to high intelligence has proceeded via a series of milestones. These include: multicellularity, bilaterality, brain, and dexterity. To what extent does evolution towards high intelligence elsewhere proceed via the same milestones? I suggest that similar steps would often be found to characterize evolution on other inhabited planets, providing it can continue for long enough. I put forward the hypothesis that there are at least a trillion radio-level intelligences in the observable universe right now. Then I consider the possible implications of ‘first contact’ between humans and one of them. Such contact could pose a threat for human survival. Finally, I look at home-grown threats, including the fixed mind-sets that underlie religious fundamentalism and science denial. I end by urging a robust defence of both science and humanity against such unthinking views.