![]() ![]() The body plans of the earliest multicellular organisms are asymmetric and do not have a head or tail. About 750 million years (My) after the appearance of multicellular life we saw the arrival of the Phylum Porifera (sponges), the most ancient metazoan (Chang et al. ![]() 2015) and over time became more specialised and diverse, giving rise to the many plants, animals and fungi we see today (Hofkin, 2010). The first multicellular organisms originated 1.5 Bya (for a detailed timeline and updated phylogenetic classification see Lecointre & Le Guyader, 2007 and revised higher order classifications by Ruggiero et al. The special characteristics of the four‐chamber mammalian heart are highlighted by the peculiar structure of the myocardial muscle. The structure and function of the circulatory system of each individual holds a vital role which benefits each species specifically. Subsequent tetrapods, including crocodiles and alligators (Order Crocodylia, Subclass Crocodylomorpha, Class Reptilia), birds (Subclass Aves, Class Reptilia) and mammals (Class Mammalia) evolved a four‐chambered heart. ![]() It has evolved from the early chordate circulatory system with a single layered tube in the tunicate (Subphylum Urchordata) or an amphioxus (Subphylum Cephalochordata), to a vertebrate circulatory system with a two‐chambered heart made up of one atrium and one ventricle in gnathostome fish (Infraphylum Gnathostomata), to a system with a three‐chambered heart made up of two atria which maybe partially divided or completely separated in amphibian tetrapods (Class Amphibia). The primitive blueprint for the circulatory system emerged around 700–600 Mya and exhibits diverse physiological adaptations across the radiations of vertebrates (Subphylum Vertebrata, Phylum Chordata). Convergence is the tendency of independent species to evolve similarly when subjected to the same environmental conditions. ![]()
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