English: Figure 2. Evolution of an archetypal animal, a presponge (vii), from a stem choanoflagellate (i–ii,v) prior to integrin loss by crown choanoflagellates. Choanoflagellates feed by catching bacteria (B) drawn by ciliary water currents (i, arrows) to their collar filters; the cell body phagocytoses them (ii). Extant craspedid choanoflagellates may be unicells (i,ii) or daughter cells may stick together by branched stalks (iii) or collar microvilli (iv) to make sessile multicells or via cell bodies to make planktonic swimming balls of cells (v). The first animal could simply have evolved (horizontal black arrow) by such a ball of cells joined laterally by cadherins settling onto a rockface (cross-hatched), differentiating non-ciliated pinacocytes for attachment and for support secreting extracellular mesohyl (turquoise) by both cell types and attached to them via pre-existing integrins (vi). This simplest presponge presumbly budded off ciliated swimming balls for dispersal (blue arrow), and probably had to evolve nutrient transfer from choanocytes to pinacocytes. (vii) Competition for filtering larger water volumes led to larger, stronger, three-layer (prototriploblastic) feeding laminas with mesenchyme cells specializing in ECM secretion sandwiched between choanocyte epithelia. Larger laminas led to divergent selection for large eggs capable of rapid cleavage and more numerous smaller sperm, both originally differentiated from choanocytes (rightmost blue arrows). As size increased, the pluripotent nonciliated mesenchyme cells differentiated into proliferative stem cells (archaeocytes: thenceforth the usual precursors of eggs, choanocytes continuing to generate sperm) and terminally differentiated cells (lophocytes) secreting collagen fibres to increase mechanical strength.