長文読解（人文系・自然科学系の文章）- 問題105

Symbiosis, the close and often long-term biological interaction between different species, represents one of nature's most fascinating evolutionary strategies. While many people are familiar with mutualistic relationships like those between bees and flowers, the complexity of symbiotic interactions extends far beyond simple cooperation. Recent research has revealed that symbiotic relationships can dramatically alter the evolutionary trajectory of participating species, sometimes leading to entirely new forms of life.
One of the most remarkable examples occurs in deep-sea environments, where certain marine worms have developed symbiotic relationships with chemosynthetic bacteria. These bacteria convert toxic chemicals from underwater volcanic vents into usable energy, while the worms provide them with a protected environment and essential nutrients. This partnership has enabled both species to thrive in one of Earth's most extreme environments, where sunlight never penetrates and temperatures can exceed 400 degrees Celsius.
Scientists now believe that symbiotic relationships may have played a crucial role in the evolution of complex life on Earth. The theory suggests that early eukaryotic cells emerged through the symbiotic merger of different bacterial species, with some becoming mitochondria and others evolving into chloroplasts. This process, known as endosymbiosis, fundamentally changed the course of biological evolution and made possible the diversity of life we see today.