The story Margulis told begins with the dawn of life on Earth. Three and a half billion years ago, nothing but bacteria lived on our hot, barren planet, and there was no oxygen in the atmosphere. Around three billion years ago, some of these bacteria evolved the ability to use energy from the sun to create food for themselves through photosynthesis. The waste product of this process was oxygen, and these bacteria produced so much of it that it dramatically changed the atmosphere.
OXYGEN: A DOUBLE-EDGED SWORD
We usually think of oxygen as essential to life, and for those that have evolved to use it, it is. But part of what makes oxygen so critical to most life also makes it dangerous. Oxygen can generate free radicals — atoms or molecules with a spare electron that makes them extremely reactive. Many of those reactions are detrimental, causing mutations and other forms of damage to living cells. For organisms that have not evolved the ability to prevent and repair this damage, oxygen can be toxic.
The oxygen poisoned many bacteria, but others evolved the ability to use it. Over many generations, some of these bacteria became dependent on oxygen to break down their food. Margulis proposed that these bacteria experienced several episodes of endosymbiosis:
- First, some amoeba-like bacteria ingested some of the bacteria that could use oxygen to break down food. Eventually, they evolved to live together, with the oxygen-users permanently installed inside the amoeba-like bacteria. With their new oxygen-using residents, the amoeba-like bacteria thrived in the oxygen-filled environment. These organisms were the ancestors of all eukaryotes, and the bacteria they ingested evolved into mitochondria.
- Next, one of these early eukaryotes ingested another sort of bacterium — a long, spiral-shaped type. Eventually, they too evolved to live together permanently, with the spiral-shaped bacteria living alongside the mitochondria in the host cell. These organisms were the ancestors of all animal cells, and the spiral bacteria they had ingested evolved into a number of important structures, like cilia and flagella, which help animal cells move around.
- Finally, some of those early animal cells ingested even more bacteria — the kind that had evolved the ability to photosynthesize — and these too evolved to live together permanently. These cells were the earliest ancestors of plants, and those photosynthetic bacteria within them evolved into structures called plastids — for example, the chloroplast — which allow plant cells to perform photosynthesis.
If Margulis was right, endosymbiosis had happened many times and played a major role in the evolution of life on Earth!