自从我们的共同祖先首次出现超过35亿年以来，地球上的生命一直在以各种速率发生变化。为了更好地理解已经发生的变化，它有助于寻找地球上生命历史中的里程碑。通过掌握过去和现在的生物如何在我们星球的历史中进化和多样化，我们可以更好地欣赏今天我们周围的动物和野生动物。第一次生命发展超过35亿年前。科学家估计地球大约有45亿年的历史。在地球形成后的近十亿年里，这个星球对生命不利。但到了大约38亿年前，地壳已经冷却，海洋已经形成，条件更适合生命的形成。第一个生物有机体是由地球茫茫海洋中存在的简单分子形成的，这些分子存在于38亿至35亿年前。这种原始的生命形式被称为共同的祖先。共同的祖先是地球上生命和灭绝所有生命的有机体。大约30亿年前，光合作用产生，氧气开始在大气中积聚。一种被称为蓝藻的生物体在大约30亿年前进化而来。蓝藻能够进行光合作用，这是一种利用太阳能将二氧化碳转化为有机化合物的过程 – 它们可以制造自己的食物。光合作用的副产品是氧气，随着蓝藻的持续存在，氧气在大气中积累。有性生殖在大约12亿年前发展，开始了进化速度的快速增长。有性繁殖或性别是一种繁殖方法，它结合并混合来自两种亲本生物的性状，以产生后代有机体。后代继承父母双方的特质。这意味着性别导致了遗传变异的产生，从而为生物提供了一种随时间变化的方式 – 它提供了生物进化的手段。
美国康奈尔大学论文代写：生物进化的手段
Since our common ancestors first appeared for more than 3.5 billion years, life on Earth has been changing at various rates. In order to better understand the changes that have taken place, it helps to find milestones in the history of life on Earth. By understanding how past and present creatures have evolved and diversified in the history of our planet, we can better appreciate the animals and wildlife around us today. The first life was developed more than 3.5 billion years ago. Scientists estimate that the earth is about 4.5 billion years old. In the nearly one billion years after the formation of the Earth, this planet is not good for life. But by about 3.8 billion years ago, the earth’s crust had cooled, the ocean had formed, and the conditions were more suitable for the formation of life. The first biological organism was formed by simple molecules found in the Earth’s oceans, which existed between 3.8 billion and 3.5 billion years ago. This primitive form of life is called the common ancestor. The common ancestor is the organism of life on earth and the extinction of all life. About 3 billion years ago, photosynthesis was produced and oxygen began to accumulate in the atmosphere. An organism called cyanobacteria evolved around 3 billion years ago. Cyanobacteria are capable of photosynthesis, a process that uses solar energy to convert carbon dioxide into organic compounds – they can make their own food. The by-product of photosynthesis is oxygen, and oxygen continues to accumulate in the atmosphere as cyanobacteria persists. Sexual reproduction developed around 1.2 billion years ago and began a rapid increase in evolutionary speed. Sexual reproduction or sex is a method of reproduction that combines and mixes traits from two parental organisms to produce progeny organisms. The offspring inherit the traits of both parents. This means that gender leads to genetic variation, providing a way for organisms to change over time – it provides a means of biological evolution.