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called the powerhouses of the cells. Without them we would not be able to move, think, or dream. Without them, the animal and insect kingdoms as we know them today would not exist.
    The symbiotic relationship, however, was a conditional one. The host cells, compelled to protect their own DNA, ensured their long-term survival by developing a membrane around their nuclei. The mitochondria, for the same reason, developed a double membrane. This genetic independence of the cell nuclei and mitochondria brings a fascinating twist to the symbiotic tale. It is well known that the genetic information in the nucleus of mammalian cells comes from both parents. What we didn’t know until very recently is that the genetic information in the mitochondria is passed on, generation after generation, by the female of the species only. In other words, the mitochondria, the powerhouses of our cells, come from our biological mothers. Why there is no contribution from the biological father is unknown, but it would seem that the genetic information, if any, which the sperm may carry regarding the mitochondria is either absent or, if not, lost or destroyed at the moment of conception. Be that as it may, the maternal link to our mitochondria has opened up a fascinating avenue into our understanding of human ancestry. With the discovery of this lineage, we are able to show that modern humans, Homo sapiens sapiens , as little as 200,000 years ago shared not only a common bloodline, but as recently as 60,000 years ago, a lineage through six or seven possible biological mothers. As humans, it would seem that we are more closely related to each other than we sometimes like to think. As for our link with animals, the evidence suggests that the mammalian bloodline goes back 100 million years. It would appear that the poetry of the brotherhood and sisterhood of all living things has become science.
    A similar symbiotic process occurred in plant cells as well, but where the new bacterial tenants (cyanobacteria) are what are known as chloroplasts—the “green stuff” of plants. Instead of using oxygen, they combine carbon dioxide with water and light to produce oxygen. As with mitochondria, chloroplasts too, have their own DNA.
    I t should therefore not be surprising to learn that other biological partnerships followed. One of the most important of these partnerships is described by the science writers John Briggs and F. David Peat in their book Turbulent Mirror as “the taking into the cell in another intrusionturned- marriage the highly mobile, corkscrew-shaped bacteria”—the spirochetes. Once again, in return for nourishment and protection, the spirochetes, or “wrigglers,” as neuroscientist and author Lynn Margulis calls them, made their sluggish hosts an offer they couldn’t refuse. They brought with them their stout cilia, or hairlike propelling strands, to act as miniature outboard motors for their new hosts. Could this have been a hint of the future legs and wings to come? Perhaps so, but not all wrigglers became propelling mechanisms. Some of them developed into microtubules within the host cell, eventually joining and elongating to become what is believed to be primitive axons and dendrites—the “business ends” of neurons, as Margulis describes them. As she suggests, it is not improbable that the growing network of connecting tubules developed into neurological tissue and later, much later, the first brains.
    Moving on to four cosmic years ago (900 million years), we would have found ourselves in the company of the planet’s first multicellular plants. Known as stromatolites from the Greek stroma , meaning “matrix” or “tissue,” they established themselves in networks of algae or algal beds. One galactic turn later we would have seen the first jelly-fish, coelenterata, and only two cosmic years ago, the trilobites—the world’s first insects. Marine and land invertebrates were developing their first shells, or exoskeletons,