What bacteria can tell about human evolution

Original author: Tara C. Smith
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To discover the deep history and shape the future health of our species, we need to learn from the microbes that accompanied us on our evolutionary journey

Human nature is characterized by a desire to know its origin. Alone, we examine our family trees to find ancestors lost in history. Together, scientists research data from a huge number of sources, from ancient fossils to modern genomes, and determine where humanity came from and how we became the kind that exists today.

Over the past decade, research in this area has undergone revolutionary changes due to a sharp drop in the cost of genome sequencing . The human genome project was launched in 1990 and cost $ 2.7 billion - about $ 100 for each sequenced genome. Today, the genome can be sequenced for $ 1000- $ 2000 , and we are already close to the long-standing goal of $ 100.

Although much of the work with the genome to date has focused on studying genetic risks to health and disease, we can explore the history of our species through genetic reconstruction. But our own genes will not necessarily tell us the whole story of our wanderings and migrations as a species or of all the risks to our health.

Therefore, in recent years, researchers have begun to look more closely at our “second genome” - microflora genes . Our microflora is all microscopic organisms living inside and outside us, playing a role in our digestion , training our immune system to respond correctly to pathogensthat make key vitamins and take the place that pathogens could take instead . Intestinal microbes are a “world within the world” that has evolved along with us, their owners, while the early ancestors of people moved from place to place, ate new food and met new animals and surrounding conditions. Our current microflora (the collective genetic material of microflora) reflects part of that deep history.

Extreme symbionts in our cells

There are several ways to extract information about human history from these organisms within us. One of them is the use of parts of our own cells, which are essentially microbial: our mitochondria . These organelles can be considered “extreme symbionts”: these are the remains of microorganisms that once lived freely, but now are part of all eukaryotes (complex cells) that produce energy and regulate metabolism.

Mitochondria retain their own DNA , separate from the nucleus cell. For many types of studies, mitochondrial DNA(mtDNA) is preferable to nuclear DNA. Unlike nuclear DNA, it is not a mixture of our parents' genetic material. Since mtDNAs are inherited exclusively from the egg and passed down through the maternal line, it looks more like a clone of your mother (and her mother, and her mother, etc.). Although eukaryotes have only one copy of nuclear DNA in one nucleus, they have many mitochondria, and therefore many copies of each mtDNA gene. Since the mtDNA genome is much smaller than nuclear DNA (it contains 37 genes instead of 20,000), it is easier to analyze.

An mtDNA analysis conducted in the 1980s concluded that humanity came from Africa, and the date of birth of the common maternal ancestor was determined at 100,000 - 200,000 years ago. And although this statement is today considered generally accepted, at that time it was controversial , as some biologists and anthropologists believed that modern people appeared as a group descended from a diverse, but interbreeding population of archaic people scattered throughout the Old World (the hypothesis of multi-regional human origin )

Microbes inside us can also help shed light on the travels of our ancestors, as they are also inherited in families and have long been associated with the human population. One example is Helicobacter pylori, a gastric bacterium that can cause ulcers and cancer of the stomach, but which many individuals can tolerate without any symptoms. H. pylori is transmitted from person to person , probably through saliva (oral), or through contact with feces, and possibly through contaminated food and water. Other species of Helicobacter colonize the intestines of mammals, indicating a long co-evolution of these types of bacteria, humans and our relatives. In the past, H. pylori probably colonized a large percentage of people , but this predominance in many countries has declined over the past hundred years due to improved sanitation and hygiene.

Studies over the past 15 years have studied the evolution of H. pylori by collecting and sequencing bacterial strains from different people from around the world. Researchers have found that H. pylori collected in Africa has the greatest genetic diversity (just like human populations in East Africa), and that, in principle, the main human migrations from this continent and around the globe can be traced by studying the genetic structure of this bacterium. Genetic analysis also indicated that the bacterium has evolved with humans for approximately 60,000 years - starting close to when modern humans began to migrate from Africa and transferred H. pylori and other bacteria along with them. Therefore, the H. pylori genome can be used to determine the evolutionary history of some human populations.

Restoring our past to their genes

But why do this if you can study human bones or the genome to get this information? For example, the same story told by the genetic data of two different organisms can be a serious confirmation of the hypothesis, especially when these organisms differ so much as people and bacteria. In addition, sometimes data for a single genome can fill in gaps in data that are not able to fill in other data. Genome data from H. pylori was able to help separate the two ethnic communities in the city of Ladakh in India, despite the fact that the genetic markers of people available at that time could not.

Today, instead of studying the only variant of the microbe, studying a large collection of all of them can better inform us about the state of the human race and where we can go. The idea of ​​the holobiont - the host and all the microbes associated with it , analyzed as the only gologen - is already taking on a certain form, while we are beginning to understand the thousands of microflora that can live inside and outside our bodies.

Our microflora not only reflects human evolution - it affects it. Thanks to the microbes associated with us, we can gain abilities that have a beneficial effect on the population. In a study from 2010it was found that in many people from Japan their intestinal microbes have a gene that allows them to produce an enzyme that helps them break down carbohydrates from algae more efficiently. This gene is not found in the intestines of people from North America, where seaweed is not among the main products.

This gene could have been obtained by the intestinal bacterium Bacteroides plebeius, probably from the marine bacteria Zobellia galactanivorans. The Japanese could eat Zobellia long ago, and it could enter their intestines either in whole or in parts, including free DNA. Because bacteria can get genes through horizontal genetic transfer, Bacteriodes could pick up this gene in the intestinal environment. Then this gene could begin to benefit both the bacteria and their host, opening up additional food sources, and therefore remained in the population due to natural selection.

Germs mismatch

Starting to understand the relationship between our microbes and our ancestors from ancient times, we can use this deep symbiosis not only to interpret history, but also to shape our future health. H. pylori can cause stomach cancer, but its ability to promote cancer seems to be a function of how well the bacterial strain matches the host. In a study on cancer of the stomach and H. pylori in Colombia, researchers found that African H. pylori strains were most likely to cause cancer in the Colombian population - but the same strains were not so carcinogenic in Africans. This observation indicates the possibility of preventing gastric cancer on an individual basis by minimizing the mismatch between the host and its bacteria.

Now, moving on to a deeper understanding of the presence and functioning of our microbes, we are beginning to understand how these long-term symbioses could affect what we are today. Recent studies have confirmed that the microflora as a whole in related organisms is much more similar than in organisms with less close affinity. Microflora as a whole can one day help us understand the evolutionary relationships of different species.

And although the auxiliary capabilities of microflora in understanding illnesses are sometimes excessively exaggerated, the most interesting thing is that our microflora will tell us about our ancestors lost in history.

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