  From fake social media accounts and generative-AI chatbots to financial
trading algorithms and self-driving vehicles, robots, bots, and algorithms are
proliferating and permeating our communication channels, social interactions,
economic transactions, and transportation arteries. Networks of multiple
interdependent and interacting humans and autonomous machines constitute
complex social systems where the collective outcomes cannot be deduced from
either human or machine behavior alone. Under this paradigm, we review recent
research from across a range of disciplines and identify general dynamics and
patterns in situations of competition, coordination, cooperation, contagion,
and collective decision-making, with context-rich examples from high-frequency
trading markets, a social media platform, an open-collaboration community, and
a discussion forum. To ensure more robust and resilient human-machine
communities, researchers should study them using complex-system methods,
engineers should explicitly design AI for human-machine and machine-machine
interactions, and regulators should govern the ecological diversity and social
co-evolution of humans and machines.
