Did sexual reproduction evolve at the same pace as mitochondrial evolution? It’s expensive:
Consider the energy a peacock uses to grow a fan-like tail to attract female peacocks. But that doesn’t seem to make sense: Sex only allows us to pass on half of our genes, and half the species (males) are incapable of having children. Change is constant, so these costs must come with benefits. The usual answer is that sexual reproduction creates new genetic material by swapping it around in each generation, separating beneficial from harmful mutations, and allowing for a flexible form of transformation. The seeds it stores may be useless today, but they will save the future from plagues, diseases, and parasites. The benefits of sex are often subtle and take generations to emerge, but the costs are serious and immediate. To understand sex, we need an explanation that goes back to the basic state of early complex organisms and the challenges they faced. Last year, Australian evolutionary biologist Damian Dowling and his colleagues Justin Havird and Matthew Hall proposed a surprising idea in the journal Bioessays.
The reason for this is a simple fact:
Single-celled bacteria and archaea (called prokaryotes) never reproduce sexually. They have some sexual-like behaviors, including the exchange of genes through physical contact—sometimes called “sexual reproduction.” They can easily be picked up by splitting in two. Many creatures, such as amoebas and armadillos, reproduce by splitting their chromosomes into gametes (like sperm and eggs), which then fuse to form new creatures. The oldest eukaryotes best preserved in the fossil record are red algae, which date back 1.2 billion years and are the earliest examples of sexual reproduction as demonstrated by gametes. One of the hallmarks of eukaryotic organisms is their cell structure, which includes not only a nucleus but also organelles, especially mitochondria, the incredibly efficient fuel cells that power our cells and provide the energy we need to survive. “Our argument is simple: eukaryotes are constrained by two traits—mitochondria and sex—and we think there’s a connection that’s been overlooked,” said Dowling, who led the research team at Monash University in Melbourne, Australia. The connection is that mitochondria are much more than just the cell’s batteries. Thousands of years ago, they isolated bacteria.
They are an example of how the human body is not truly “human.”
Our DNA is full of ancient bacteria; even if our brains were soupy to begin with. Scientists are increasingly realizing that many diseases are not caused by external aggressions but by imbalances in our internal ecosystems. In the mitochondria, however, a mismatch can occur because these organelles have their own unique and independent DNA. “Until recently, the scientific community largely ignored the fact that we carry two genomes in our brains,” Dowling said. “One is our own genome, and the other is the mitochondrial genome. It is sensitive to rapid change and can be easily disrupted. It can interact with gene regulators in the cell nucleus, causing adverse effects in the body. Dowling believed that the emergence of sex was a way for the cell to adapt to a changing world governed by the nucleus. “The empire that early eukaryotes built with their main weapon, the energy-producing mitochondria, was dangerous because the mitochondria mutated so often,” he says. Sex creates new genotypes in each generation and allows the nucleus to compensate in case of adverse events. In other words, it is a way of restoring balance within ourselves and improving divisions. Unlike other benefits of sex, this one was vital for early eukaryotes and their descendants. It paired in a way that could be compared to early behavior.
One attacked the other.
One ate, the other ate, but both survived. They come together and over time create something extraordinary and new. The invader, the eaten creature, has evolved over millions of years into small but powerful mitochondria. The other will be transformed into a larger base. Mitochondria are dedicated to generating energy, and they do so with great efficiency, so life on Earth spread rapidly in all directions. But the focus on generating electricity comes at a cost: Mitochondria are subject to high levels of oxidative stress, which damages organelles and genes. Dowling believes that mitochondrial DNA is therefore “bound to cause evolutionary problems.”
Plants themselves were once free-floating organisms.
Recent work by Nils-Göran Larsson of the Max Planck Institute for the Biology of Aging in Cologne, Germany, suggests that mitochondrial replication is (except in special cases) inherently error-prone. Two prokaryotes—two organisms floating in a pre-evolutionary soup—engaged in something resembling early sexual reproduction.
One ate, the other ate, but both survived.
They come together and over time create something extraordinary and new. The invader, the eaten creature, has evolved over millions of years into small but powerful mitochondria. The other will be transformed into a larger base