动物界中除原生动物外所有其他动物总称为后生动物（后生动物亚界）。其特征是体躯由大量形态有分化、机能有分工的细胞构成；与群体原生动物的兼有营养和生殖功能的细胞不同，其生殖细胞和营养细胞有明显的分化。
美英考古学家最近在安曼沉积岩中发现的化学化石为动物生活提供了迄今所发现的最早证据。这一化石类固醇（24-isopropylcholestanes，为寻常海绵纲的海绵所特有）来自距今约6.35亿年或更早以前的马林诺冰期，这是新元古代末期大冰期中的最后一个。这表明，在寒武纪大爆炸期间两侧对称动物迅速分化之前至少1亿年，一些晚成冰期海盆中的浅水中就含有浓度大到足以支持简单多细胞生物的溶解氧。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 457, 718-721 (5 February 2009) | doi:10.1038/nature07673
Fossil steroids record the appearance of Demospongiae during the Cryogenian period
Gordon D. Love1,2, Emmanuelle Grosjean3, Charlotte Stalvies4, David A. Fike5, John P. Grotzinger5, Alexander S. Bradley2, Amy E. Kelly2, Maya Bhatia2, William Meredith6, Colin E. Snape6, Samuel A. Bowring2, Daniel J. Condon2,7 & Roger E. Summons2
1 Department of Earth Sciences, University of California, Riverside, California 92521, USA
2 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 01239, USA
3 Petroleum and Marine Division, Geoscience Australia, Canberra, Australian Capital Territory 2601, Australia
4 School of Civil Engineering and Geosciences, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK
5 Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA
6 School of Chemical, Environmental and Mining Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, UK
7 Present address: NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK.
The Neoproterozoic era (1,000–542 Myr ago) was an era of climatic extremes and biological evolutionary developments culminating in the emergence of animals (Metazoa) and new ecosystems1. Here we show that abundant sedimentary 24-isopropylcholestanes, the hydrocarbon remains of C30 sterols produced by marine demosponges, record the presence of Metazoa in the geological record before the end of the Marinoan glaciation (635 Myr ago). These sterane biomarkers are abundant in all formations of the Huqf Supergroup, South Oman Salt Basin, and, based on a new high-precision geochronology2, constitute a continuous 100-Myr-long chemical fossil record of demosponges through the terminal Neoproterozoic and into the Early Cambrian epoch. The demosponge steranes occur in strata that underlie the Marinoan cap carbonate (>635 Myr ago). They currently represent the oldest evidence for animals in the fossil record, and are evidence for animals pre-dating the termination of the Marinoan glaciation. This suggests that shallow shelf waters in some late Cryogenian ocean basins (>635 Myr ago) contained dissolved oxygen in concentrations sufficient to support basal metazoan life at least 100 Myr before the rapid diversification of bilaterians during the Cambrian explosion. Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, 713 Myr ago in Oman2).
