**2. The origin of helminths**

Helminths are free-living parasitic invertebrate metazoan organisms. They include nematodes (round worms), trematodes (flukes), cestodes (tapeworms), and acanthocephalans (thorny-headed worms). The fossil record provides evidence that ectoparasitic helminths (e.g., worm-like pentastomid arthropods) have existed since the early Paleozoic era (542–444 million years (My)), while endoparasitic helminths (cestodes) arose during, or possibly even before, the late Paleozoic era (416–251 My) [4]. Therefore, the origins of helminths, all from free-living and parasitic organisms, were derived from a world in which the atmospheric conditions were initially reductive before transforming to oxidative [5].

### **3. The "rusting" of the Earth**

The amount of oxygen (O2) in the atmosphere before the Paleozoic era was at levels <0.001% of those present in the atmosphere today. However, during the Paleozoic and after this era, free oxygen was spawned by cyanobacteria producing land releasing it as a by-product of photosynthesis [6], causing the Great Oxidation Event (GOE), which dramatically changed the composition of the Earth's life forms and led to the near extinction of anaerobic organisms. The GOE is believed to have input sufficient oxygen into the atmosphere to allow for the evolution of animal respiration **Figure 1**.

On the other hand, if cyanobacteria were fundamental for the "rusting" of the Earth, redox reactions (electron transfer mechanism or redox) would still be relevant, in particular for the physiology of aerobic organisms.

*Oxygen and Redox Reactions Contribute to the Protection of Free-Living and Parasite Helminths… DOI: http://dx.doi.org/10.5772/intechopen.102542*

#### **Figure 1.**

*Earth atmosphere modification and consequences on living organisms. Cyanobacteria are associated with the Great Oxidation Event (GOE) on Earth. Then redox reactions contribute to the development of reactive oxygen, nitrogen, and sulfur species (ROS, RNS, RSS). High concentrations of these are avoided through glutathione (GSH)/thioredoxin (TrxR) systems, but low species concentrations are necessary for signal transduction pathway cells to control gene expressions.*

In the Hadean eon (4.6 billion years ago), redox reactions were a response to the large amounts of energy in the primitive Earth resulting from cosmic and geophysical reactions occurring at the time [7].
