Humans are made of billions of cells from "summary" of A Short History of Nearly Everything: Special Illustrated Edition by Bill Bryson
It is a startling thought that every human being begins life as a single cell, a fertilized egg. From this tiny, undifferentiated blob comes everything that makes us human - the bones and blood, the muscles and brain. But the real wonder is not just that we start life this way, but that we continue to exist in this way. Indeed, for most of our lives, we are no more than a collection of cells. Each of us is made up of trillions of cells, each one carrying out its own particular function. The cells of the heart beat in unison, the cells of the brain transmit electrical signals, the cells of the skin protect us from harm. It is a complex and intricate system, a symphony of cells working together to create the miracle of human existence. And yet, despite the incredible diversity of cells in the human body, they all share certain basic characteristics. They all have a nucleus, where the genetic material is stored. They all have a membrane, which regulates what goes in and out of the cell. And they all have mitochondria, the tiny powerhouses that produce the energy the cell needs to function. But perhaps the most remarkable thing about cells is not what they have in common, but what makes them unique. Each cell in the human body contains a complete set of instructions for building a human being. These instructions are encoded in our DNA, the master molecule that governs every aspect of our biology. And yet, despite the incredible complexity of our genetic code, every cell in our body contains a copy of it.- When we say that humans are made of billions of cells, we are not just talking about the stuff of life. We are talking about the very essence of what it means to be human - a vast, interconnected network of cells, each one playing its own crucial role in the grand symphony of existence. It is a humbling thought, and a reminder of the incredible complexity and beauty of the world within us.
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