Which animals mate for life?

The origin of the first animal on Earth is a fascinating and complex scientific question. To answer the question "What was the first animal on Earth?" we need to go back hundreds of millions of years in geological time and biological evolution. Through fossil records, genetic analysis, and studies of existing organisms, scientists have been able to speculate what the first animal on Earth might have been like, and how it emerged and

1. Time of the Origin of Animals

The Earth formed about 4.6 billion years ago, and life probably first appeared about 3.5 billion years ago. Early life forms were mainly tiny, simple, single-celled organisms, such as bacteria and archaea. However, complex multicellular organisms, including animals, did not appear until the Precambrian period, about 600 to 700 million years ago.period

As for the appearance of the first animals, current research points to the Ediacaran period, about 600 to 700 million years ago.（Ediacaran Period），This is the period in Earth's history when multicellular life began to emerge in large numbers. Prior to this period, life on Earth was mainly single-celled, and the Ediacaran period saw the rise of the first multicellular organisms, which became the ancestors of later complex life forms.

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1. Fossil Evidence

Sponges are considered the earliest animals. The latest research uses high-energy technology to analyze massive genetic data and finds that comb jellies separated from other animals before sponges and established their own evolutionary path. The study is based on two new evolutionary hypotheses, arguing that comb jellies have developed complexity independent of other animals after separating "organisms" and forming their own evolutionary pathways. Although this discovery needs more research to confirm, it challenges the traditional view that sponges are the earliest animals to separate from the tree of life‌1.

In addition, some studies have proposed another possibility that sponges may be the earliest animals on Earth. A study published in the journal Nature pointed out that a Canadian geologist discovered 890 million-year-old sponge fossils, which may significantly advance the fossil record of the earliest animals on Earth‌2. However, this discovery has caused controversy.

Recent studies have also discovered the earliest known bryozoan fossils on Earth in Zhenba, southern Shaanxi. This discovery pushed the origin of the phylum Bryozoa forward by at least 50 million years, to the early Cambrian explosion‌3. This further supports the hypothesis that the Cambrian explosion was the most magnificent life explosion event of bilaterally symmetrical animals known on Earth.

Other studies have found fossils of Dickinsonia, a creature that dates back to the Ediacaran period. Their fossils have been found in strata dating back 558 to 555 million years ago around the world. This is considered to be the first animal on Earth with clear fossil evidence.

2. Genetic Analysis

Based on genomic research, scientists have compared the DNA of existing animals and found that the genes of sponges have an ancient common ancestor with the gene sequences of other complex animals. Genetic research shows that the ancestors of sponges may be the first multicellular organisms to differentiate from single-cell ancestors, making them the base of the animal evolution tree.

3. The simplicity of biological structure

The simplicity of sponges also makes them an ideal model for studying the earliest stages of animal evolution. They have no complex tissues or organ systems and are composed of a group of cells with different functions, which is very similar to the characteristics of early multicellular animals.

3. Ctenophores: A possible older candidate

In addition to sponges, comb jellies

1. Challenges of genetic evidence

In 2013, a groundbreaking genetic study pointed out that comb jellies may have appeared earlier in the history of animal evolution than sponges. The study found through genome sequencing that comb jellies lack some genes commonly found in other complex animals, indicating that they may be another early branch parallel to sponges.

2. Complexity of the nervous system

Although ctenophores appear complex, with nervous and muscular systems, their nervous systems are significantly different from those of other animals. The nervous systems of ctenophores may have evolved independently, further complicating their evolutionary status.

3. Differences from other animals

Although comb jellies share symmetry and locomotion with other complex animals, their embryonic development is different from that of most other animals. This feature makes comb jellies a very special branch of animal evolution, and may represent an evolutionary path that is completely different from that of existing animals.

4. Common ancestor of animal origin

Although sponges and comb jellies are considered to be among the earliest animals on Earth, they are not the most primitive multicellular organisms. The first true animal on Earth may be the common ancestor of these complex animals, that is, the primitive multicellular organism .。

1. Evolution of protozoa

Before sponges, comb jellies, and other complex animals appeared, life evolved from single cells to multicellular organisms. Scientists believe that this process may have been through the formation of single-celled organisms into colonies, which gradually evolved into groups of cells that could cooperate and differentiate. This colony may have evolved over millions of years, eventually developing into multicellular structures with specific functions.

2. The earliest multicellular organisms

By studying the simplest multicellular organisms in existence, such as choanoflagellates（

3. Characteristics of early multicellular organisms

These early multicellular organisms may not have complex organ systems or nervous systems, and their life activities mainly depend on the coordination between cells. Through natural selection, these early organisms gradually evolved more complex functions, such as differentiating into different cell types, and eventually formed the prototypes of modern animals.

5. Environmental Factors of Animal Origin

The origin of animals is not just a biological issue. Changes in the Earth's environment are also a key factor in animal evolution. The early Earth experienced a series of dramatic climate changes that created suitable conditions for the evolution of life.

1. Increase of oxygen

About 800 million to 600 million years ago, oxygen levels on Earth rose significantly. This phenomenon, known as the "oxygen revolution," provided the necessary conditions for the evolution of complex life. The emergence of multicellular animals required higher oxygen concentrations because they needed oxygen for metabolism and to maintain complex physiological functions.

2. Ice Ages and Evolutionary Pressure

Before multicellular animals appeared, the Earth experienced a long ice age, known as the " Snowball Earth. "”This event may have exerted strong evolutionary pressure on life, forcing organisms to adapt to new environmental conditions. Scientists speculate that it was in this extreme environment that early single-celled organisms began to form colonies and eventually evolved into multicellular organisms.3. Marine environment

The environment in which early animals lived was mainly the ocean. The ocean is rich in nutrients and has a relatively stable climate, providing ideal living conditions for early multicellular organisms. Today, we still find many ancient biological species in the ocean, and the genes of these species are similar to those of early animals, helping us trace the origin of animals.

VI. Conclusion

There is no simple answer to the question of what the first animal was on Earth, but through the fossil record, genetic studies and analysis of environmental changes, scientists are getting closer to the mystery. Sponges are widely believed to be among the first multicellular animals, and comb jellies may also be a very ancient branch of animals. However, the real first animal may be the common ancestor of these complex creatures, a primitive multicellular organism that lived in the Earth's oceans about 600 million to 700 million years ago.

Whatever the first animal was, its appearance marked an important turning point in the history of life, opening the way for the evolution of complex life forms. These early animals were not only the ancestors of modern animals, but also the basis for the development of biodiversity on Earth.

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