Glyph origin or Word Etymology Resources
zdic.net is an authoritative resource: http://www.zdic.net/z/16/xs/53D6.htm

会意。从又,从耳。甲骨文字形。左边是耳朵,右边是手(又),合起来表示用手割耳朵。古代作战,以割取敌人尸体首级或左耳以计数献功。本义:[捕获到野兽或战俘时]割下左耳。

However, I don't think it is true that 取 developed from 聝 and 馘, they have always been separate characters, even though Shuowen (same site) indicates that 取 has also been used in that sense: 捕取也。从又从耳。《周禮》：“獲者取左耳。”《司馬法》曰：“載獻聝。”聝者，耳也。七庾切。