Origins Institute, Department of Physics and Astronomy, McMaster University , Hamilton, Canada .
Astrobiology. 2018 Mar;18(3):343-364. doi: 10.1089/ast.2017.1674. Epub 2018 Mar 12.
Estimates of the time at which life arose on Earth make use of two types of evidence. First, astrophysical and geophysical studies provide a timescale for the formation of Earth and the Moon, for large impact events on early Earth, and for the cooling of the early magma ocean. From this evidence, we can deduce a habitability boundary, which is the earliest point at which Earth became habitable. Second, biosignatures in geological samples, including microfossils, stromatolites, and chemical isotope ratios, provide evidence for when life was actually present. From these observations we can deduce a biosignature boundary, which is the earliest point at which there is clear evidence that life existed. Studies with molecular phylogenetics and records of the changing level of oxygen in the atmosphere give additional information that helps to determine the biosignature boundary. Here, we review the data from a wide range of disciplines to summarize current information on the timings of these two boundaries. The habitability boundary could be as early as 4.5 Ga, the earliest possible estimate of the time at which Earth had a stable crust and hydrosphere, or as late as 3.9 Ga, the end of the period of heavy meteorite bombardment. The lack of consensus on whether there was a late heavy meteorite bombardment that was significant enough to prevent life is the largest uncertainty in estimating the time of the habitability boundary. The biosignature boundary is more closely constrained. Evidence from carbon isotope ratios and stromatolite fossils both point to a time close to 3.7 Ga. Life must have emerged in the interval between these two boundaries. The time taken for life to appear could, therefore, be within 200 Myr or as long as 800 Myr. Key Words: Origin of life-Astrobiology-Habitability-Biosignatures-Geochemistry-Early Earth. Astrobiology 18, 343-364.
关于地球上生命起源的时间估计，主要利用了两类证据。第一，天体物理学和地球物理学的研究提供了地球和月球形成的时间尺度，大的撞击事件对早期地球的影响，以及早期岩浆海洋的冷却时间。从这些证据中，我们可以推断出一个可居住性边界，即地球最早具有可居住性的时间点。第二，地质样本中的生物特征，包括微化石、叠层石和化学同位素比值，为生命存在的时间提供了证据。从这些观测结果中，我们可以推断出一个生物特征边界，即最早有明确生命存在证据的时间点。通过分子系统发生学和大气中氧气含量变化的记录的研究提供了额外的信息，有助于确定生物特征边界。在这里，我们回顾了来自广泛学科的数据，总结了这两个边界时间的最新信息。可居住性边界最早可能出现在 45 亿年前，这是地球具有稳定地壳和水圈的最早可能时间估计，或者最晚可能出现在 39 亿年前，即重陨石撞击期结束的时候。对于是否存在足以阻止生命的晚期重陨石撞击，缺乏共识，这是估计可居住性边界时间的最大不确定性。生物特征边界的约束更为严格。碳同位素比值和叠层石化石的证据都指向接近 37 亿年前的时间。生命一定是在这两个边界之间出现的。生命出现所需的时间可能在 2 亿年以内，也可能长达 8 亿年。关键词：生命起源-天体生物学-可居住性-生物特征-地球化学-早期地球。天体生物学 18，343-364。
