It’s not harmful to us at least not immediately. But it’s harmful to science. The Outer Space Treaty text calls it “harmful contamination” and that’s been taken as including harmful to the science experiments of the parties to the treaty.

To understand the value, you need to realize what it is that scientists might find there. It could be the most significant breakthrough in biology of the twentyfirst century. Future historians of biology might talk about Darwin’s theory of evolution in the nineteenth century, the helical structure of DNA and understanding of how DNA works in the twentieth century, and discoveries of exobiology in the twentyfirst century.

So - we just should not mess that up. Not when we don’t know what there is that we could discover. Not through ignorance. We have made so many other mistakes and it’s well within our capabilities to make a mistake as big as this.

The rest of this is from one of my kindle books. MOON FIRST - Why Humans on Mars Right Now are Bad for Science.

WHAT WE COULD DISCOVER

What we discover there could include any of:

• Early life, e.g. tiny RNA world microbes without DNA or proteins. There are many ideas for early life that could perhaps still exist there, though extinct on Earth. These could fill in the huge gap between the organics and cell like structures resembling cells that turn up in laboratory experiments, and the immense complexity of modern life. One idea is an RNA world cell with no proteins, or ribosomes either, instead using RNA sliced into pieces and recombined to make a ribozyme, a tinier distant cousin of the ribosome. This is possible in theory, and some have suggested that present day Earth might have a "shadow biosphere" consisting of RNA world cells, but this has never been confirmed.Maybe we can find RNA world cells on Mars instead?
  
  There are many other ideas for early life that could perhaps still exist there, though extinct on Earth, including the so called autopoetic cells that replicate just by producing daughter cells with a similar mix of chemicals when they get large, with no genetic code to regulate the process.
• Unrelated life, perhaps based on some form of XNA (Xeno Nucleic Acid) instead of DNA. This would be the most amazing discovery of all. It would lift biology into a new dimension, show how life can exist based on completely different principles from DNA based life.
  
  There are many alternatives to DNA and RNA. RNA and DNA are both particularly fragile, DNA especially and hard to form naturally, need the environoment of the cell or special conditions to keep them stable. RNA is more stable when it is very cold for instance, and ribose in its backbone is stabilized by the presence of borates, one of the points in favour of an origin on Mars. Some of the others are more robust and some think we may have started with a PNA world for instance as it is far more robust than RNA and forms more easily.
  
  Other ideas for early life include TNA world, or a molecule that's a hodgepodge mixing different backbones in the same molecule with non heritable variations in backbone structure (or a whole alphabet soup" of other possible precursors such as HNA, PNA, TNA or GNA - Hextose, Peptide, Therose or Glycol NA).
  
  The interior of a cell is so complex it's been compared to an entire ecosystem. So life based on different principles could be as revolutionary for biology as discovering a coral reef for your first time, when the only ecosystem you knew about before is the African Savannah. I make this analogy here: "Super Positive" Outcomes For Search For Life In Hidden Extra Terrestrial Oceans Of Europa And Enceladus
• Life that is based on novel new principles that we haven't thought of yet. For instance, what if other life doesn't use a helix? Suppose for instance that the life used a sheet like two dimensional structure, planar rather than linear, and replication happened by a second layer forming on top of the original sheet?
  
  Or could it even be a 3D informational polymer? Is there any approach that avoids the need to uncoil to read it? We can do this mechanically through laser scanning, in prototypes for future memory devices, so the idea is not so far fetched as to be totally impossible.
  
  This is just fun speculation at present. But suppose that you are an ET biologist and your life uses 2D sheets to replicate - would you not find the idea of a helical structure that has to uncoil and unzip to replicate implausible and unlikely too?
• Life that has evolved further than Earth life. Mars has had such harsh conditions in the early solar system, alternating ice and more habitable phases. It's also been subject to strong ionizing radiation, extremes of cold, and near vacuum atmosphere. Some think that we have multicellular life on Earth as a result of a snowball Earth phase. If that's true, you could make a case for Mars life to be more highly evolved than Earth life - more complex, more robust cells, with more non redundant nucleotides, and more capabilities than Earth life, maybe even totally novel capabilities never explored here, even if it is just single cell life.
  
  Present day Mars probably only has microbes, or perhaps lichens, if it is fair to make a comparison with similarly harsh environments on Earth. But the harsh environment may mean it evolved further on Mars than on Earth. Or could mean it didn't get as far and is an early form of life. It's hard to say in advance which way this would go
• Life with a capability Earth life doesn't have, e.g. a new form of photosynthesis.
  
  We have three ways of doing photosynthesis on Earth - broadly speaking.
  
  Green sulfur bacteria, which use light to convert sulfides to sulfur, which is then often oxidized to sulfur dioxide
  Normal photosynthesis which splits water to make oxygen, also taking up carbon dioxide in the process. (basic equation 6CO2 + 12 H2O → C6H12O6 + 6O2 + 6 H2O where the oxygen atoms in bold are the same ones on both sides of the equation - see Plants don't convert CO2 into O2, and Notes on lamission.edu)
  The photosynthesis of the haloarchaea which works similarly to the receptors at the back of our eyes, based on a "proton pump" which moves hydrogen ions across a membrane out of the cell using bacteriorhodopsin similar to the rhodopsin in our eyes, with no byproducts such as sulfur or oxygen, just creates energy directly from the proton gradient.
  
  ET microbes might well use some fourth form of photosynthesis that has never been explored on Earth.
• Life similar to Earth life in most respects, would raise many questions. How has it evolved in such a different environment, since last transfer from Earth, surely at least tens of millions of years ago. How did it get there? We can test the theory of panspermia, find out in practice how easy it is for life to be transferred to another planet.
• No life but with organics, and all the ingredients for life but no life. This may seem boring, but it would tell us a lot about how hard it is for it to evolve on a planet, and about the paths it follows on the way to life. If not life itself, there has to be some complex organic chemistry going on, and cell like structures surely form, as that happens even in short term laboratory experiments. So how far did it get and what exactly happens on a world similar to Earth in many ways (especially in the early solar system), but without life?
  
  Also, on Earth it's impossible to study uninhabited habitats, except for a very short time after a volcanic eruption. Life appears rapidly on any uninhabited habitat here. On Mars, we might have the opportunity to study uninhabited habitats on a planet that hasn't been inhabited for billions of years. This could help us to understand exoplanets and the origin of life and maybe find out that life is harder to evolve than we thought. It can also help to disentangle effects of life and non life processes on Earth.
• Some major unexpected discovery that nobody currently is likely to predict.

It might seem hard to get excited about microbes - but think of them as microbe ETs, and perhaps you may see them in a different light. As minute emissaries from another biological cosmos, tiny beings with a potentially totally different biochemistry.

This next video shows how DNA makes protein. Notice how complex the process is.

It happens in exactly the same way in every cell of every single Earth creature. Imagine what it would be like to find a cell that does it differently?

I've heard it said that the interior of a cell is so complex, with its million different chemicals, and elaborate structures and processes, that to researchers studying how cells work, it seems as complex as an entire ecosystem. So, what about using actual ecosystems as an analogy here?

Imagine that you have been brought up in the African savannah - with its grasses and trees, elephants and antelopes. You've never seen a marsh or a forest, or a beach. All your life you've lived in a hut in the African Savannah, never traveled more than a few miles from your hut, and that's the only thing you've ever known.

View of Ngorongoro from Inside the Crater

Then one day someone takes you to the sea shore, with its fish, shellfish, seaweeds, and sea anemones, and perhaps they take you on a dive to see a coral reef.

A Blue Starfish (Linckia laevigata) resting on hard Acropora coral. Lighthouse, Ribbon Reefs, Great Barrier Reef. Photo by Richard Ling

The interior of a cell of XNA based life could be as different from the interior of a cell of DNA based life as the African Savannah is different from a coral reef. And imagine the new perspectives we might get if we can study it.

The search for life is the main motive for all the missions to Mars to date. Look at how excitedly NASA reports yet another discovery of possible past or present water on Mars. And what a huge anticlimax it would be to get there, find life, it's headline news in all the papers, and then follows the anticlimactic announcement that it was just life that was brought there by the human explorers themselves! Then would follow speculation and questions about whether there was any native Mars life there before the Earth microbes got there, maybe never answered definitively. Or we find evidence that there was some native life that went extinct in the very decade that humans landed there, an ecosystem of many Mars microbes interacting, now gone. Or we find some present day indigenous life, but it is already getting overwhelmed by microbes from Earth, and there follows a rush to try to find it in the many different potential habitats on Mars before it goes extinct.

I think the example of an early form of life is the easiest to use here to show how vulnerable native Mars life could be, potentially. It could be some form of life that was been made extinct on Earth billions of years ago, RNA world life say. It might not last for long after more modern life from Earth gets to Mars.

For more about this see my article Will We Meet ET Microbes On Mars? Why We Should Care Deeply About Them - Like Tigers

See also my MOON FIRST - Why Humans on Mars Right Now are Bad for Science available to read online for free

(Also on kindle)