**1. Introduction**

A crucial moment in the evolution of the planet was the change from an anoxic primordial atmosphere to one rich in oxygen (O2). Currently, it is accepted that this change began with the origin of cyanobacteria and the development of oxygenic photosynthesis [1,2] and continued later with the emergence and diversification of photosynthetic pigments in different types of algae and plants [3,4]. The latter allowed the atmosphere to accumulate oxygen over millions of years and, after some fluctuations in the Carboniferous period, to reach its current level (around 21%) [5,6]. In turn, this process influenced the evolution of life on Earth, since geological and fossil evidence has allowed us to infer that the increase and accumulation of O2 in the atmosphere gave rise to the establishment of specific ecological niches. Hence, some of the organisms adapted and developed in the increasingly aerobic conditions, whereas others established themselves in a microaerophilic environment and still others in sites where fully anaerobic conditions predominated [7,8]. There is even a proposal that considers that the first organisms to emerge were anaerobes, which allowed them to adapt and eventually live in hypoxic conditions [9,10]. In fact, it is recognized that anaerobic glycolysis is an ancestral metabolic pathway as it is present in these first living beings, which in turn allowed them to produce ATP at substrate-level phosphorylation in the absence of O2 [11,12]. Moreover, the late oxygen accumulation in the atmosphere caused the emergence of aerobic-type energy metabolism, in which the now available O2 is the final electron acceptor, enabling the ability to obtain a greater amount of ATP from a glucose molecule. This would favor the appearance and diversification of new metabolic pathways [13], which in turn enabled organisms to evolve into more complex forms.

**Citation:** Martínez-González, J.d.J.; Guevara-Flores, A.; del Arenal Mena, I.P. Evolutionary Adaptations of Parasitic Flatworms to Different Oxygen Tensions. *Antioxidants* **2022**, *11*, 1102. https://doi.org/10.3390/ antiox11061102

Academic Editor: Serge Ankri

Received: 29 March 2022 Accepted: 29 May 2022 Published: 31 May 2022

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Oxygen is one of the two main products derived from oxygenic photosynthesis. This molecule is chemically interesting since it has two unpaired electrons in the anti-union orbitals (with the same spin), which makes it difficult for it to oxidize another molecule and accept two electrons simultaneously [14]. This restriction in molecular oxygen (that is, the dioxygen di-radical) significantly decreases its reactivity; however, exposure to physical factors such as high temperatures or some source of radiation can cause a change in the spin, thereby decreasing said restriction, which favors the acceptance of one electron at a time (that is, oxygen becomes more reactive), making reactions very slow [15]. Due to this, an incomplete reduction of O2 can occur, generating a series of molecules that, when accumulated, can cause adverse effects in the cell. These molecules are generically called reactive oxygen species (ROS); they include the superoxide radical anion (O2 •−), hydrogen peroxide (H2O2), and the hydroxyl radical (HO•), among others.

The purpose of this review is to analyze some of the basic adaptations presented by parasitic flatworms, a specific group of organisms that face changes in the concentration of oxygen (and related molecules) throughout their life cycle.
